An Etymological Dictionary of Astronomy and Astrophysics
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A parallel of latitude on the Earth, 23°26' north of the equator, where the Sun is directly overhead on the northern → summer solstice (around the 21st June each year), because the Sun reaches its most northerly declination. Some 3,000 years ago, this occurred when the Sun was in the → Zodiac constellation → Cancer, hence the name. However, → precession has resulted in a shift of the position of the Sun so that it is now in the constellation → Gemini on the summer solstice.

→ tropic; → Cancer.

A parallel of latitude on the Earth, 23°26' south of the equator, where the Sun is directly overhead on the southern → summer solstice (around the 21st December each year), because the Sun reaches its most southerly declination. Some 3,000 years ago, this occurred when Sun was in the → Zodiac constellation → Cancer, hence the name. However, → precession has resulted in a shift of the position of the Sun so that it is now in the constellation → Sagittarius on the northern → winter solstice.

→ tropic; → Capricorn.

1) Geography: Pertaining to, characteristic of, occurring in, or inhabiting the → tropics.
2) Astro.: A misnomer for → solar, as in → tropical year, or → vernal equinox, as in → tropical month.

→ tropic + → -al.

The average period of the revolution of the Moon about the Earth with respect to the → vernal equinox, a period of 27.321 582 days (27d 7h 43m 4.7s).

→ tropical; → month.

Mâh, → month; hamugâni, of or pertaining to hamugân, → equinox.

The interval during which the Sun's mean longitude, referred to the mean equinox of date, increases by 360 degrees. Its mean length for the epoch J2000.0 is 365.24217879 real solar days (approximately 365.2422 days). This concept of tropical year, adopted by the International Astronomical Union at its General Assembly in Dublin, September 1955, has often been confounded with the → vernal-equinox year. In fact the mean period between two successive true vernal equinoxes is different from the tropical year. This period, which is equal to 365.24236460 solar days (about 365.2424 days), is the real mean length of the year in the Iranian calendar. The difference between the two year lengths is due to the fact that the Earth's orbital velocity around the Sun is not uniform, since the orbit is an ellipse. At the perihelion of its orbit the Earth is closest to the Sun, and therefore moves faster than average, while at aphelion, when it is farthest away from the Sun, it moves slower. Therefore the interval between two successive vernal equinoxes is not the same as the period between two successive summer solstices. In fact the tropical year does not depend on a specific origin for the annual apparent motion of the Sun. For detailed discussion see: A concise review of the Iranian calendar.

→ tropical; → year.