Astronomy Answers: From the Astronomical Dictionary

# Astronomy AnswersFrom the Astronomical Dictionary

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The description of the word you requested from the astronomical dictionary is given below.

the year

The year is a period of time that is related to the motion of the Earth around the Sun. There are quite a few different kinds of years in use:

• the calendar year: a period of (depending on the used calendar) between 354 and 384 days. Some calendars (such as the Gregorian calendar that is used in most developed countries) follow the seasons without regard to the phases of the Moon, and have 365 or 366 days in any given year. Other calendars (such as the Islamitic calendar) follow the phases of the Moon without taking into account the seasons. Yet other calendars (such as the Jewish calendar) follow both the phases of the Moon and the seasons.
• the Julian year: a period of 365¼ days, which is the average length of a calendar year in the Julian calendar.
• the sidereal year: the period after which the Sun returns to the same position (ecliptic longitude relative to a fixed equinox) between the stars, as seen from Earth.
• the tropical year or solar year: the period after which the seasons return, when the Sun returns to the same position along the ecliptic, relative to the vernal equinox (the same ecliptic longitude relative to the equinox of the date). The length of the tropical year depends on the ecliptic position that the year is tied to, and on whether the true Sun or the mean Sun (that ignores small disturbances) is meant.

For the four cardinal directions along the ecliptic, tied to the beginning of the four seasons, the tropical years are on average as shown in the following table. In this table, the "length" is the corresponding ecliptic longitude, "month" the month in the Gregorian calendar when the Sun reaches the cardinal direction, "year" the length of the corresponding tropical year, "change" the rate of change in the length, and "name" a name that I propose for this type of year.

Table 3: Year of Seasons

length month "year" change name
degrees Gregorian days sec/century
0 march 365.24237 +0.893 ascending-equinox year
90 june 365.24163 +0.056 northern-solstice year
180 september 365.24202 −2.000 descending-equinox year
270 december 365.24274 −1.075 southern-solstice year

Sometimes, tropical year is used as the name for what I call the ascending-equinox year above (the year defined by the ascending equinox, which is the vernal equinox).

Sometimes, tropical year is used for the average over all positions along the ecliptic (the average over all seasons). We can call that the "average tropical year", though that is still ambiguous. Do you mean averaged over all positions along the ecliptic (for one or more elapsed years), or averaged over a number of years (for one position along the ecliptic)? For more distinction, we could call the first kind the "ecliptic-averaged tropical year", and the second the "time-averaged tropical year" (but we should then also specify the associated position along the ecliptic, e.g., the "time-averaged northern-solstice year").

On average, the ecliptic-averaged tropical year now lasts 365 days, 5 hours, 48 minutes and 45.20 seconds, and decreases in length by 0.53 seconds per century.

It is clear that "tropical year" by itself is too vague. If the difference between all of the subtypes is important to you, then you should explain very clearly exactly which kind of tropical year you mean.

• the anomalistic year: the period after which the Earth is again closest to the Sun (in the perihelion), or furthest from the Sun (in the aphelion). This year is now 365.25964 days long and increases in length by 0.1365 seconds per century.