A Stellar Cycle

An alternative to measuring the passage of the sun through a fixed point (equinox or solstice) would be to measure the appearance of a particularly prominent star or constellation, either just before sunrise or just after sunset. These events are called heliacal rising and setting and will yield a period that is fairly close to the mean tropical year. Such a measurement might seem like more work, but it can be handy, particularly if you live in someplace like the desert or a small island where the horizon is too flat to make a reliable measurement of the changes in the sun's rising position. Old Hawaiian religious ceremonies, for example, began each year with the heliacal rising of the Pleiades. Over a long time, however, a calendar based on heliacal rise and set times will slip with respect to the solar equinox—this is the phenomenon known as the precession of the equinox and is the result of the top-like wobble of the Earth's axis, which takes about 25,800 years to complete one full wobble. A year based on this kind of measurement is known as a sidereal year (sidus = Latin for "star"), and is about 51.15 seconds longer than the tropical year.