The Sun's Zodiacal Motion & the Two Kinds of "Year" Is it confusing to talk about two kinds of solar year? Don't panic because they are both about 365 1/4 days long (give or take about 20 minutes). (1) Tropical (Seasonal) Year The tropical year is the interval between two successive solstices or equinoxes of the same type; e.g., between two successive summer solstices. The tropical year is essentially a measure of the intervals between the seasons that occur on planet earth (thus the alternative term of "seasonal" year makes sense). The modern value for the length of the tropical year is about 365.2422 days. (1) Sidereal (Zodiacal) Year The sidereal year (Latin for "starry year") is the amount of time it takes for the Sun to move back in alignment with a given star. Because stars in the zodiac constellations are some of the most conveniently located stars by which to judge this cycle of the Sun (they are near the sun's path along the background of fixed stars), the term "zodiacal year" makes sense). ). The modern value for the length of the sidereal year is about 365.2564 days. The difference between the tropical and sidereal years can be easily calculated as follows: 365.2564 - 365.2422 = 0.0142 days. To convert this answer to minutes we would do as follows: 0.0142 days * 24 hours/1 day * 60 minutes/1 hour = answer in minutes. To solve this equation for minutes, we would multiply the following numbers together: 0.0142 * 24 * 60 = 20.448 minutes. The difference between tropical and sidereal years is subtle, and was not recognized until Hipparchos conceptualized it as the "precession of the equinoxes" in the second century BC See explanation in Crowe, pp. 8&endash;10.