Astronomy Answers
AstronomyAnswerBook: Constellations

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This page answers questions about constellations. The questions are:

• [599] Can multiple meteors appear in the same constellation within a short time?
• [566] I the first quarter moon found in the same constellation each month?
• [527] Can stars move to another constellation?
• [508] What is the best way to divide the starry sky into constellations?
• [507] How many constellations are in the Zodiac?
• [506] Why can the vernal point move into the constellation of Aquarius but not into the sign of Aquarius?
• [505] Is there a standard table with the size of all 12 zodiacal constellations, in degrees, minutes, and seconds?
• [493] How much overlap is there between neighboring constellations?
• [492] Aren't the boundaries of the constellations as defined by the IAU very subjective? Aren't there older and better, more intuitive definitions?
• [488] Are there calendars that are tied to constellations?
• [450] What color, element, and stone are associated with the constellation of the Crab?
• [406] What is the mythological explanation for the constellation of the Crab?
• [275] How far are the constellations from us?
• [265] Why do constellations and the Sun and the Moon appear larger when they are close to the horizon and smaller when they are overhead?
• [226] What is the size of the constellation Scorpius?
• [216] Which constellation reaches furthest from the Earth?
• [215] Are the stars of our constellations in other galaxies?
• [154] Why can you see some constellations all year round, but not others?
• [141] When is the Sun in which constellations?

You can find questions about how constellations move in the sky on the page about the Sky and Horizon.

1. Constellations

1.1. What is a Constellation?

A constellation is

1. a set of stars in the sky of which someone thinks that they form a pattern that looks like a certain person or animal or object.
2. a certain area in the sky that contains one such set of stars.

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All constellations are to a high degree arbitrary. Anybody can invent their own constellations, and in different countries and different times many different constellations have been invented that often had little or nothing to do with the constellations that were known in other countries or times. There is no single "truth" about constellations. If it were obvious how to define the constellations, then we'd already all be using the same definitions.

1.2. IAU Constellations

Eugène Delporte in 1930 divided the sky into 88 constellations and that division has been adopted by the IAU (see //www.iau.org/IAU/Activities/nomenclature/const.html), and those constellations are usually mentioned in modern star atlases.

The boundaries that Mr. Delporte defined consist of straight line segments that have either constant right ascension or constant declination. The advantage of building boundaries out of line segments of constant right ascension and constant declination is that this makes it easier to check on which side of such a line segment a particular star is. For example, one of the segments of the boundary between Aquarius and Pisces is defined as "declination equal to exactly −4 degrees, for right ascensions between exactly 22h45m and exactly 23h50m (relative to the equinox of 1875)", with Aquarius below and Pisces above that boundary. So, once I've reduced the coordinates to the equinox of 1875, I can tell whether a position between those right ascensions is in Aquarius or in Pisces by comparing its declination with the value −4. All I need to do is to compare the right ascension and declination to various fixed values; I don't need to do any complicated calculations. It is difficult to find a simpler method. The algorithm to determine the constellation from the right ascension and declination needs only 1428 values (1071 numbers and 357 [partially duplicate] constellation names) to cover the whole celestial sphere, or about 16 values per constellation. A similar algorithm to determine the country on Earth from the longitude and latitude would need a vastly greater number of values to cover the whole sphere, because boundaries on Earth are a lot more crooked.

The IAU divisions were an attempt to formalize the division of the sky into constellations in such a way that all of the stars that had already been assigned to particular constellations by tradition up to that point would end up in those same constellations also using the IAU rules. The tradition up to that point was (at least for the northern stars) based on the constellations as defined by the Ancient Greeks, so (nearly?) all stars that the Ancient Greeks of 2000 years ago recognized are assigned to the same constellations following the IAU rules as they were by the Greeks themselves. The same probably holds for the star names assigned by Johann Bayer (around 1600, like "α Centauri") and John Flamsteed (around 1720, like "51 Pegasi"), and for the names of variable stars (such as "R Andromedae").

There were probably a few (much) weaker stars that were assigned to one constellation by some people but to some other constellation by other people, so there the IAU had to choose between alternatives that could not satisfy everyone.

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1.3. The Best Definition of Constellations?

There is no single "truth" about constellations. If it were obvious how to define the constellations, then we'd already all be using the same definitions. Instead, different peoples in different parts of the world have come up with very different ways to divide the night sky into constellations. The definitions given by the IAU are "merely" a convention, but a useful one, because all serious publications about astronomy use that same convention. Nevertheless, you are welcome to define your own constellations, but you mustn't hope that many people will use your definitions.

This situation is quite similar to the problem of how to name the various plants and animals, or geological layers, or planets, or medical conditions and ailments. There are no inherently best names for those things, and in fact different people in different times and places have invented very different names for the same things, and different peoples may have used the same names but for different things.

The best way to make it easy for people all over the world to use the same names for the same things is to have some professional, respected organization define some convention for naming them. People are not forced to follow that convention, but if two people use the same convention then they'll be able to discuss those things a lot more easily, and the more people use the same convention, the greater the chances will be that your conversation partner knows the same convention as you. Because of these benefits, such a convention usually becomes more popular as time goes by, which increases its use, which increases its popularity, and so on. Usually at least the professionals in a given field all use the same convention for naming the things that are important in their field. Such conventions are known by the general name of "nomenclature" (//en.wikipedia.org/wiki/Nomenclature).

Some people imply that it is possible to define constellations in a better, more "intuitive" way than the IAU has done. I don't believe that there are "intuitive" boundaries to the constellations, except if by intuitive you mean personal, i.e., not the same as those defined by anyone else. If I give 100 people a picture of the night sky (stars only) and ask them to draw the boundaries between the constellations, then it is unlikely that any 2 of them will draw the exact same boundaries ― even if they are familiar with star maps that do show boundaries between constellations. Certain areas would be more likely to receive a boundary than others, but there would always be some differences between the boundaries drawn by different people.

The constellations defined by the IAU are highly subjective, but also very precise. For any point in the sky between the stars, and for any star, we can determine exactly in which IAU constellation it lies. If you want your constellations to be very precise, then you must make very subjective decisions about where exactly to put the boundaries.

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The constellations defined by the IAU have zero overlap. Each point in the sky belongs to exactly one IAU constellation. It would be confusing if some points in the sky belonged to more than one constellation at the same time, just like it would be confusing if some points on Earth belonged to more than one country at the same time. Suppose that there were overlap between the constellations of Pisces and Aquarius. Should we then say that the Moon enters Pisces when it first enters the zone of overlap (the first moment when it is in Pisces, but also still in Aquarius), or when it leaves the zone of overlap (the first moment when it belongs to Pisces alone), or when it is half way through the zone of overlap, or something else again? Different people might choose different solutions. Overlap is confusing.

The ancient Greeks from before our era mentioned many of the northern constellations that the IAU also uses, but constellations were then defined by lists of specific stars that belonged to those constellations, and not in terms of specific boundaries between the constellations. Dim stars between the specific stars of different constellations were regarded as "loose stars" or "scattered stars" that did not belong to any particular constellation. It seems likely to me that all other people from those long-ago times also defined their constellations in terms of lists of stars rather than in terms of boundaries.

1.4. Table of Constellations

The constellations defined by the IAU are listed in the following table. The table shows for each constellation: the English name, the official (Latin) name, the official abbreviation, and a list of all the neighbors of the constellation. Two constellations are neighbors when they have a common boundary that is more than just a point. The constellations are sorted into alphabetical order of their official Latin name.

Table 1: Constellations and Their Neighbors

The following table shows characteristics of each constellation. The characteristics are:

• \( α \) = the right ascension (in degrees relative to the equinox of J2000.0) of the weigthed average of the position of all stars in the constellation.
• \( δ \) = the declination (in degrees relative to the equinox of J2000.0) of the weigthed average of the position of all stars in the constellation.
• \( λ \) = the ecliptic longitude (in degrees relative to the equinox of J2000.0) of the weigthed average of the position of all stars in the constellation.
• \( β \) = the ecliptic latitude (in degrees relative to the equinox of J2000.0) of the weigthed average of the position of all stars in the constellation.
• \( l \) = the galactic longitude (in degrees) of the weigthed average of the position of all stars in the constellation.
• \( b \) = the galactic latitude (in degrees) of the weigthed average of the position of all stars in the constellation.
• \( w \) = an estimate for the diameter of the constellation, measured by the weighted conjunction spread (in degrees) of the stars in the constellation.
• \( B \) = the total of the brightness of each star in the constellation (in units of the brightness of a star of magnitude 0) over the brightness of a star of magnitude 6. Stars of magnitude 6 or dimmer do not contribute.
• \( A \) = the solid angle or area (in square degrees) covered by the constellation.

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• \( d \) = the weighted average distance (in lightyears) of the stars in the constellation. The mean distance is the weighted harmonic mean of the distances of the stars, so the inverse of the mean distance is the weighted mean of the inverse of the distances of the stars. Each constellations contains stars that are closer by and stars that are further away. The average gives most weight to the brightest stars.
• \( M \) = the total brightness of the visible part of the Milky Way in the constellation. The units are chosen so that the total for all constellations is equal to 1000.
• \( O \) = the date (month-day) in the Gregorian calender on which the center of the constellation (\( α, δ \)) is furthest from the Sun and therefore most visible.

All values except for the total brightness are weighted with the brightness that each star has over the brightness of a star of magnitude 6, so bright stars contribute more than dim stars. All values are based on data from the Hipparcos catalog.

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For example: the constellation Scorpio (Sco) has a size of 497 square degrees.

The average declination \( δ \) of a constellation indicates from which parts of the world that constellation can be seen. From the northern hemisphere of the world, all constellations can be seen that have a positive declination, and also constellations with negative declinations if those are not more negative than your northern latitude minus 90 degrees. From the southern hemisphere, all constellations can be seen that have a negative declination, and also constellations with positive declinations if those are not greater than 90 degrees minus your southern latitude. For example: From 40 degrees north latitude, you can see constellations with declinations between −50 and +90 degrees, and from 40 degrees south latitude you can see constellations with declinations between −90 and +50 degrees.

The average right ascension \( α \) of a constellation determines (together with other things) in what time of the year a particular constellation is best visible. Around 9 o'clock pm (local solar time) each January the constellations with right ascensions between about 4 and 6 hours are best visible, in February the right ascensions between about 6 and 8 hours, and each next two hours of right ascension for each next month. The column \( O \) shows the date on which the constellation culminates at midnight (local solar time). For example, for Orion this date is 12-15 (December 15th), so Orion is best (longest) visible around the middle of December.

The average ecliptic latitude \( β \) of a constellation shows how far it is, roughly speaking, from the path of the Sun (the ecliptic). The constellations of the zodiac have an average ecliptic latitude that is close to 0.

The average galactic latitude \( b \) of a constellation indicates how far that constellation is, roughly speaking, from the plane (central line) of the Milky Way. If the galactic latitude is close to 0, then the constellation is close to or even in the Milky Way in the sky.

1.5. The Largest and Smallest Constellations

The five largest and smallest constellations are listed in the following table, with their size in square degrees. The five largest constellations together cover about 15 percent of the heavens (above and below the horizon). The five smallest constellations together cover 1 percent of the heavens. For comparison, the full moon covers about 0.2 square degrees, so the smallest constellation is about 350 times larger in the sky than the full moon. The Netherlands covers about 2.8 square degrees on Earth.

1.6. The Brightest and Dimmest Constellations (Regarding Bright Stars)

The five brighest and dimmest constellations are listed in the following table, together with their total brightness \( B \) as defined above. The five brightest constellations together cover 27 percent of the brightness of all constellations together, which is 63.8 units. The five weakest constellations cover 0.4 percent of the total. For comparison: the brightness of the full moon is about 150,000 units, and the brightness of Venus when that planet is brightest is about 50 units.

1.7. The Farthest and Nearest Constellations

The following table lists the five constellations that are nearest on average and the five that are furthest on average, as defined above, with their average distances measured in lightyears. These distances are only averages that take the brightest stars most into account. Each constellation has stars that are closer than the average, and many stars that are further away (of which most are dim and contribute little to the average). The average for all stars (with emphasis on the brightest stars) is 94 lightyears.

1.8. The Brightest Constellations (Regarding the Milky Way)

The five brighest constellations are listed in the following table, together with their total brightness \( B \) as defined above. The five brightest constellations together cover 45 percent of the brightness of the Milky Way in the sky.

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2. The Crab

The Crab is an ancient constellation. The earliest Greek philosophers of over 2000 years ago already mentioned the constellation. According to mythology ([Allen]), the crab pinched the toes of Hercules when Hercules was fighting the Water Snake (Hydra). Hercules then squashed the crab, and the goddess Juno then put the crab in the sky.

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3. Larger near the Horizon, Smaller Overhead

The constellations and the Sun and the Moon seem to be larger when they are close to the horizon than when they are overhead, because your brain plays tricks on you. The constellations aren't really larger when they are close to the horizon. You can check this for yourself if you compare the size of the constellations with (for example) the size of your fist when you extend your arm fully, or if you compare the size of the Moon with the width of your thumb when you extend your arm fully. Check this when the constellation or Moon is high in the sky, and also when the constellation or Moon is low in the sky. Their size is the same in both situations.

I think that a constellation seems large near the horizon and small overhead because you unconsciously compare it to different things. When it is low in the sky, then you compare its size with that of far-away things on Earth, such as a building or mountain near the horizon, and then the constellation or Moon seems to be quite large by comparison. When it is overhead, then you see the Moon or constellation in the middle of an even much bigger sky, and then it seems small by comparison.

Try to estimate how large (in inches or centimeters or feet or meters or kilometers or miles) the Moon is when it is close to the horizon and when it is high in the sky. I expect that the answer is smaller when the Moon is high in the sky than when the Moon is near the horizon.

I did not mention refraction in the explanation I just gave. Refraction lifts up the image of a celestial object near the horizon, and the more the closer the object is to the horizon. Refraction can only have a systematic effect in the vertical direction, because the atmosphere is layered only in the vertical direction. It is impossible to make everything appear, for example, twice as large in the horizontal direction, because if that happened everywhere along the horizon, then the horizon would have to be twice as large in circumference, and that doesn't fit. So, the image of the Sun (the solar disk) is equally wide at every height above the horizon.

The effect of refraction in vertical direction can be seen in the Sun or Moon when they are low in the sky, because then the Sun and the Moon appear a little squashed, because the bottom is lifted up more by refraction than the top (because the bottom, as long as it is visible, is closer to the horizon than the top). The Sun appears to be 15% flatter when the bottom of the solar disk touches the horizon. When the bottom of the Sun is still 1 degree (two diameters) above the horizon, then the flattening is 10%. If the Sun is 5 degrees above the horizon, then the flattening is only 2.5%.

So, refraction close to the horizon does not make the image of the Sun or Moon larger, but rather smaller, because it is flattened in the vertical direction. The effect is at most 15%, and in the wrong direction, so it cannot explain the "small when high, large when low" effect, which works in the other direction, does not change the shape, and appears much greater than 15%.

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4. Constellations and Galaxies

All constellations visible from Earth are made up of stars from just the nearest part of our Galaxy. Most if of the stars that make up the constellations are less than 1000 lightyears from Earth, while the diameter of our Galaxy is about 100,000 lightyears.

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The question which constellation has the widest range of distances of its stars is about as difficult to answer as the question which section of the beach has the widest range of sizes of the grains of sand. It is impossible to say exactly which constellation has the widest range of distances of its stars, unless you specify very carefully which stars can be considered and which cannot. If one star is small but close by and another star is large but far away, then they can still appear equally bright in our sky. If you use better equipment, then you can see stars that are dimmer and further away.

The Galaxy is much more extended in the directions where you can see the Milky Way at night than in other directions, so constellations that contain part of the Milky Way (such as Cassiopeia and the Swan and the Archer) have a better chance of including stars that are really far away than constellations that are far away from the Milky Way (such as the Great Bear or Leo).

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5. Ecliptic and Zodiac

The Sun moves against the background of stars and constellations like the center of a merry-go-round moves against the background of its surroundings when you sit on its edge while it rotates. It depends on the relative position of the Earth and the Sun where the Sun appears to be among the stars. Because the Earth always orbits around the Sun in the same plane, the Sun seems to traverse always the same path between the stars. That path is the ecliptic. The Moon and bright planets orbit around the Sun in about but not quite exactly the same plane as the Earth. They can therefore get up to a few degrees away from the equator (the Moon up to about 5 degrees, for example). The region of the starry sky where the Moon and planets can be is called the zodiac.

It is hard to tell where the Sun is relative to the stars, because you cannot usually see any stars when you see the Sun. One way to find out is to wait for a total solar eclipse, when you can see the stars near the Sun. Another way is to look which stars reach their highest point in the sky exactly twelve hours after the Sun is there (i.e. at midnight local solar time - not midnight on the clock): the Sun must then be at exacly the opposite side of the ecliptic.

The astronomical constellations of the zodiac are not the same as the astrological signs of the zodiac with the same name. The signs of the zodiac were fixed about 2500 years ago by the Babylonians. They divided the ecliptic in twelve equal parts and named the parts according to the constellations that they recognized at that time. The signs of the zodiac were linked to times of the year according to where the Sun was at those times. Since that time, two things have happened to make the astronomical constellations and the astrological signs get out of step:

1. The astronomical constellations and their boundaries have been officially fixed by the International Astronomical Union in 1930, and they do not always coincide exactly with the constellations that were recognized by the Babylonians. In particular, the modern division of the sky into 88 constellations does not give special consideration to the ecliptic. The astronomical constellations through which the ecliptic goes do not all contain equal parts of the ecliptic, and the ecliptic goes through thirteen rather than twelve constellations (and passes within 5 degrees of three more constellations, so the Moon and some planets can traverse those constellations).
2. Astronomers tie the constellations to the stars, but astrologers tie the signs of the zodiac to the seasons, and the stars and seasons don't run exactly in step. The Sun enters the sign of Aries (the Ram) when it passes through the vernal equinox, which is at the beginning of spring of the northern hemisphere, and each next sign starts about a month later. At the time of the Babylonians, the Sun was in their constellation of Aries (the Ram) during the period of the astrological sign of Aries, but because of the precession of the equinoxes, the vernal equinox moves relative to the stars (and constellations) by one degree about every 71.6 years. The vernal equinox, and hence also the signs of the zodiac, have moved by about 35 degrees (or just over one astrological sign) since the time of the Babylonians. So, nowadays at the vernal equinox (i.e., the beginning of the period of the astrological sign of Aries), the Sun is about 35 degrees away from what the Babylonians considered to be the start or the defining point of the constellation of Aries.

When the Sun is in a particular astrological sign of the zodiac, then nowadays the Sun is not in the astronomical constellation of the same name. The signs of the zodiac are not connected to the stars, but to the seasons.

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6. Constellations of the Zodiac

The following table lists some information about the astronomical constellations through which the ecliptic goes. The astronomical information is derived from [1] and from my own calculations; the astrological information from [2]. Note: there are twelve signs of the zodiac, but the ecliptic passes through thirteen constellations. The various columns contain the following information:

1. name = the Latin name of the constellation or sign.
2. deg = the size of the part of the ecliptic (of J2000.0) that is in that constellation, measured in degrees. The astrological signs each cover exactly 30 degrees, but there is no sign of Ophiuchus.
3. days = how many days the Sun spends in each of the constellations. The Sun spends about 30 days in each astrological sign.
4. in = the approximate date of the year, in the Gregorian calendar, that the Sun enters the constellation. These dates may be off by about a day from year to year, and move back by about one day per 71 years because of the precession of the equinoxes.
5. astrol = the approximate first day (in the Gregorian calendar) of the astrological sign of the zodiac. These dates may be off by about a day from year to year, too, but do not change with the precession of the equinoxes. If your web browser can show them, then for each sign of the zodiac the corresponding symbol is displayed after the date. Note: there is no astrological sign of Ophiuchus.
6. vernal = the first year (approximately) of the period in which the vernal equinox has been or will be in the corresponding constellation.

Also included in the table are the dates of the year of the solstices and equinoxes (the beginnings of the seasons). These, too, may vary by a day, but they are not noticeably affected by the precession of the equinoxes over a few thousand years (in the Gregorian calendar).

Table 2: Constellations of the Ecliptic

Notes:

(1) The Sun approaches Orion to about 2 degrees during this period.

(2) The Sun approaches Sextans to about 2 degrees on this day.

(3) The Sun approaches Cetus to less than 0.5 degrees on this day: part of the Sun but not all of it enters Cetus.

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6.1. How Many Constellations are in the Zodiac?

The zodiac is the part of the starry sky through which the Sun, Moon, and planets move. I have calculated for a million days (about 2739 years), with the year 2000 in the middle, in which constellation the Sun, Moon, and planets then stand, as seen from the center of the Earth (if the Earth were made of glass). The row with title '#' shows in how many constellations I found the celestial objects during that period. The other numbers in the table show during how many days out of each 1000 days (on average) that celestial object was in that constellation. For example, the Sun passed through 13 constellations, and was in the constellation of Aquarius (Aqr) on average 67 out of every 1000 days.

Table 3: Planets in Constellations

#   

The amount of time that each planet spends in a particular constellation is not the same for all planets, because the orbits of the planets have different inclinations (are tilted to different degrees) and because the orbits are not circles (so the planets traverse certain parts more quickly, and other parts more slowly).

We see that Venus can stand in 23 different constellations, the Moon in 18, and the other planets in at least 14. Pluto (a dwarf planet) can stand in a few constellations where none of the regular planets can be, because of the large inclination of the orbit of Pluto.

If we define the zodiac as the set of constellations through which the Sun, Moon, and regular planets (i.e., not Pluto) can move, then that zodiac contains at least 23 constellations. At least, because perhaps you'll find one more if you do the calculations for more than one time per day − but such an additional constellation will then be traversed very seldomly, just like Pegasus (Peg).

Of all constellations, the Virgin (Virgo) is the one in which the regular planets (no dwarf planets) spend the most time (between 11% and 14%).

The planet Venus spends a small amount of time in a few constellations. Venus was in the Charioteer (Auriga) on 22 and 23 April of the year 781. Venus will be in the Little Dog (Canis Minor) on a few days in August/September of the years 2557, 2792, 3035, 3270, and 3278. Venus travels through Pegasus once in a while. The last time was in March 1806, and the next times will be in March 2025, 2033, and 2041. Venus will pass through the Shield (Scutum) in January 2014 and again in February 2492. Venus spends a few days in the Water Snake (Hydra) in August/September every 8 years between 1983 and 2015. Venus passes through the Cup (Crater) once in a while. The last time was in August/September 1911, and the next time will be in October 2106. Venus spends some time in the Crow (Corvus) occasionally. The last time was in September 1871, and the next time will be in October 2058.

The Moon is very seldomly in the constellation of the Crow (Corvus): only on one day each of the years 782, 819, 1545, 1805, 1898, 1935, and 1991.

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6.2. Constellation versus Sign

Imagine a rope with both ends tied together. Along the rope there are 12 knots spaced at equal distances. One of those knots is extra large and represents the vernal equinox. Each of the segments between one knot and the next one represents an astrological sign.

Now put the rope unto some ground covered in little stones, so that the rope forms a circle. The little stones represent the stars, and interesting groups of them represent constellations. The circle that the rope traces between the little stones represents the ecliptic (or zodiac). About 2500 years ago people defined the signs and gave them the names of nearby constellations. The first sign is called Aries, because that segment of the ecliptic (rope) passed through (or near) the constellation (group of stones) called Aries. The 11th sign is called Aquarius, because that segment of the ecliptic passed through (or near) the constellation Aquarius.

As time passes, precession makes the ecliptic (rope circle) rotate along itself, in such a way that it still traces out the same path between the stars (stones) but now the vernal equinox is in a different place, similar to how the valve moves relative to the axis of the wheel of a bicycle if you turn the wheel: the wheel is still in the same place relative to the bicycle, but now the valve is in a different position.

In the last 2500 years, the vernal equinox has moved over about 35 degrees or so, compared to the stars. All of the signs (segments of the rope between two knots) are at a fixed location relative to the vernal equinox (the big knot), so when the vernal equinox moves over 35 degrees compared to the stars (stones), then all of the signs move over 35 degrees as well.

So, now the sign (rope segment) called Aquarius is no longer very near the constellation (group of stones) called Aquarius. The vernal equinox is now closer to the constellation (group of stones) of Aquarius than it was before, but the vernal equinox is still equally far from the sign (rope segment) called Aquarius, because the vernal equinox and the sign moved together over the same distance.

6.3. Changing the Ecliptic

The precession doesn't only let the vernal equinox shift roughly parallel to the ecliptic, but also tilt the ecliptic a bit. This means that a star that today lies exactly on the ecliptic might not lie exactly on the ecliptic anymore tomorrow, because the ecliptic has tilted away from that location. With this, the length of the segment of the ecliptic that lies in each constellation also changes. The next table shows the lengths of the segments of the ecliptic in each constellation, for 1 January in steps of 1000 years between the years 0 and +3000, measured in degrees (calculated by me, based on [Meeus]). I believe these numbers to be accurate to about one unit in the last decimal.

The part of the ecliptic in the constellations Ari (Ram), Leo (Lion), Vir (Virgin), Sco (Scorpion), and Aqr (Water Carrier) shrinks during this period. The part in the constellations Tau (Bull), Gem (Twins), Cnc (Crab), Lib (Scales), Oph (Serpent Bearer), and Sgr (Archer) grows. The part in the constellation Cap (Sea Goat) rose to a greatest length equal to 27.831492° in the year 1893, and is shrinking since then.

7. The Age of Aquarius

The New Age concept of the "Age of Aquarius" and of other signs of the zodiac is linked to the movement of the vernal equinox through the corresponding constellation. If you go by the astronomical constellations, then the boundaries of the constellations are well-defined, so the period when the vernal equinox is in a given constellation can be fairly accurately determined. In this case, the current "Age of Pisces" started around 68 BC and lasts until about AD 2597, when the "Age of Aquarius" starts (i.e., the vernal equinox moves into the astronomical constellation of Aquarius). The details of the boundaries of the modern constellations are quite arbitrary (though now well-defined), and astronomers assign no special importance to whatever constellation the vernal equinox happens to be in.

If you want to go by equal-length zodiacal constellations, then each age has about the same length, of some 2148 years, but then it is not clear where each of the equal-length constellations starts. Different sources give different periods for the same age: I have seen claims for the start of the New-Age "Age of Aquarius" early in the 19th century AD ([2]), and around AD 2570 (derived from a stated Age of Taurus between about 3880 and 1730 BC, [3]).

A correspondent reports that there is a religious organization based on the assumption that the Age of Aquarius began in 1948. That is only possible if their version of Aquarius has different boundaries than those defined by the IAU.

Scientific studies have shown that astrological predictions that are based solely on the position of stars and planets and such in the sky on average do not fit the people for whom they were made any better than they fit other people. Scientists don't consider the signs of the Zodiac or the New-Age Ages to be important.

8. References

• [1] "Redshift 2" CDROM, Maris Multimedia.
• [2] "Encyclopaedia Brittannica" CDROM.
• [3] Richard Hinckley Allen: "Star Names: Their Lore and Meaning", Dover Publications, Inc., New York, 1963 (1899), ISBN 0-486-21079-0.

9. Precession of the Equinoxes

The rotation of the Earth around its axis, which causes the alternation of day and night on its surface, is similar to that of a spinning top. The rotation axis of a spinning top itself rotates around the vertical direction, and in a similar way the rotation axis of the Earth changes its orientation, but only very slowly. The top of a spinning top traces out small circles in the air as the rotation axis changes direction, and the rotation axis of the Earth traces out a curve in the sky that is almost a circle, too. It takes about 26,000 years for the rotation axis to complete one cycle.

Because of the change in the rotation axis, the point in the sky among the stars where the Sun is at the beginning of spring (the vernal equinox) and the other seasons changes around, too. This is called the precession of the equinoxes, and it changes the relation between the stars and the seasons, i.e., which stars you can see in what season, and their times of rise and set.

The most common coordinate systems in the sky (for measuring the position of stars and planets and such; e.g., the equatorial and ecliptic coordinate systems, but not the galactic or horizontal systems) are linked to the position of the vernal equinox, so even if a particular celestial object does not move at all its coordinates in those coordinate systems slowly change with time. This means that it is important to specify the time for which the used coordinate system is valid. This time is called the equinox, for instance "the equinox of 2000.0".

The official boundaries of the constellations were defined for a particular, fixed equinox, I think the one of 1875. If you want to know what constellation a particular direction in the sky belongs to, you first have to translate its coordinates to the equinox of 1875, i.e., to the coordinate system that was valid in 1875. This means that the precession of the equinoxes cannot be the reason why a star moves from one constellation to another one.

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10. Stars to Another Constellation

Stars can move from one constellation to another one because of their proper motion: their motion along the sky because of their motion through space relative to the other stars. The proper motion of the brightest visible stars is commonly measured in milli-arcseconds per year and the size of constellations in tens of degrees, so a typical star takes a couple of millions of years to cross a constellation.

The boundaries of constellations have mostly been drawn so that the brightest stars (those with Bayer or Flamsteed designations that have a constellation name in them, such as α Andromedae or 82 Ursae Maioris) are not real close to any of them, so for most bright stars it will take a very long time before they move to another constellation.

To get some idea of how often stars move across a boundary between different constellations, I calculated for all stars from the Hipparcos catalog in which constellation they are between the years 1000 and 3000, and notes which stars moved to another constellation. The Hipparcos catalog contains information about the position and motion of over 100,000 stars. It is assumed to contain all stars down to magnitude 7.3, and an increasingly smaller fraction of dimmer stars.

I found that in that period of 2000 years 218 stars from the Hipparcos catalog cross a constellation boundary, which means on average about one star per 10 years. This rate depends a lot on how many stars are in the catalog. Of the constellation switchers (between the years 1000 and 3000), 3 are brighter than magnitude 4, 15 brighter than magnitude 6, and 32 brighter than magnitude 7.

The next table shows the constellation switchers between the years 1850 and 2150. \( Δ \) is an estimate for the uncertainty in the date (measured in years), which follows from the uncertainty in the position and proper motion of the stars. "HIP" and "HD" give the numbers of the stars in the Hipparcos catalog and the Henry Draper catalog. \( V \) is the visual magnitude. Then follow the Latin abbreviations of the constellations in which the star is before and after the switch, and then the Bayer name and/or proper name of the star (which not all stars have).

Because the boundaries of the constellations are defined in the equatorial coordinate system of B1875.0, the date on which a star changes its constellation depends also on the translation to B1875.0, which depends on the model that is used for the precession. If you use a different precession model, then you'll get different results. I used the precession model given by [Meeus], and B1875.0 = JD 2405889.25855.

Table 4: Stars Move to Other Constellations

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According to these calculations, the star Elnath has been in the constellation of the Bull only since 1867. That star is nowadays called β Tau, but in the past also used to be called γ Aur. In 1994 the star ρ Aql moved from the Eagle (Aql) to the Dolphin (Del). Other bright stars that switch constellation between the years 1000 and 3000 are: τ₁ Eri (came from Cet in 1184), 58 Oph (came from Sgr around 1364), 18 Sco (from Oph in 1609), α Cen B (from Cir around 1763), α Cen A (from Cir in 1771), γ₁ Cae (will move to Col around 2397), γ₂ Cae (to Col around 2620), ε Ind (to Tuc around 2642), ε Scl (to For around 2942).

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Last updated: 2021-07-19