Star Points for January, 2002; by Curtis Roelle Which Happy New Year? When January arrives on the heels of December we commonly greet each other with wishes for a "Happy New Year." This is because we know that in our Gregorian calendar system, established in 1582, January is the start of another numbered year. But do you realize how many different kinds of years are in use today? A popular publication containing descriptions for may types of years is Guy Ottewell's "The Astronomical Companion" published by the Universal Workshop at Furman University (universalworkshop.com). It is a sort of non-fictional Hitchhiker's Guide to the Galaxy. This month we take a look at several of the years found in Ottewell's useful book. First it should be pointed out that a year is simply a measurement of the length of time. In our civilian calendar we assign numbers to each "calendar year" starting on January 1. However, a calendar year can be used to denote the length of time between any two dates such as from one birthday to the next. A typical calendar year has 365 days. Nearly every four years we insert an extra day producing a 366 day leap year. This is necessary because the period of time it takes the Earth to revolve around the Sun is not an integral number of days. Although each calendar year is either 365 or 366 days long the average length is 365.2425 days. The actual time it takes Earth to make a full trip around the Sun, making a complete 360 degree circle with respect to the stars, is called the "sidereal year." One sidereal year equals 365.2564 days. Before going much further it should be mentioned that the "day" being used here is the "ephemeris day" or a period of exactly 86,400 seconds. That is, twenty-four 60 minute hours, with 60 seconds in each minute. No surprise there, I hope. From the point of view of an observer on Earth the first day of Spring occurs as the Sun appears to cross northbound across the celestial equator (Vernal Equinox). Other celestial crossing points mark the start of each of the other three seasons as well. These imaginary seasonal points slowly move due to a wobbling motion of the Earth's axis known as precession. Thus, the period of time from the start of one season until the start of the same season next year is 365.2422 days. This Equinox-to-Equinox period is called the "tropical year." A less common name for the tropical year is the "seasonal year." The Earth does not orbit the Sun in a perfectly circular motion. It moves in an ellipse and so the distance separating the two varies. The point at which the Earth is closest to the Sun is called "perihelion." Like the Equinox this perihelion point precesses as well. However, the perihelion point precesses for different reasons and in the opposite direction as the precessing Equinox. The period of time from one perihelion passage to the next is the "anomalistic year" or 365.2596 days. Armed with this yearly overview you're prepared to beat others to the punch next New Year's Day with a hale and hearty "Happy New Calendrical Year!" Only three days later as the Earth reaches it's annual perihelion point on January 4 surprise them again with "Happy Anomalistic New Year!" Four times a year as the seasons change your friends will cringe in anticipation of hearing your "Happy New Tropical Year" greeting.