Now that the Winter Solstice is past, days are getting longer again and things are settling down after the Christmas and New Year’s celebrations, we are starting to look forward to spring. One thing we can do is watch the eastern sky in the late evening for the first signs of the spring constellations. For me the herald of spring is the appearance in the east of Arcturus, a bright, golden star in the constellation of Boˆtes, “The Herdsman”. Many astronomy books promote the idea of particular constellations with particular seasons. That idea is roughly right but misleading.
The daily and seasonal rhythms we see in the sky are due to the Earth’s rotation and its orbiting around the Sun. A day is the average interval of time between two successive times when the Sun is in the south, and the year is how long it takes the Earth to complete a trip around the Sun. However, the easiest way to picture what is going on is to use an analogy put together by mediaeval astronomers, based upon exactly what they saw in the sky.
They imagined the Earth to be spinning in the middle of a great “celestial sphere”, with all the stars fixed to it, and the Sun, Moon and planets moving around on it. It’s clear that over one day, anyone standing on the Earth will see the all the constellations visible from their latitude.
As the Earth orbits the Sun, we see our star against the constellations behind it. Over a year, we see different parts of the celestial sphere in the background, with their own particular constellations. Our astronomical ancestors actually plotted the path of the Sun against the constellations. That path, which changes only very slowly with time, is called the ecliptic. It passes through thirteen constellations. We don’t like thirteen, so we forget one of them (Ophiuchus), and call the remaining twelve the Zodiac.
In principle, the only time it is hard to see the stars is when the Sun is above the horizon. Fortunately the apparent motion of the Sun against the background stars, together with our clocks being related to the Sun, cause any given star to rise about four minutes earlier each night. That means a star rising at 10 p.m on December 1 will rise at 8 p.m. on January 1. For example, in the summer, just before the Sun comes up, you will see the sky filled with what we believe to be the stars of winter. By autumn they are rising around midnight and by Christmas they are in the sky by the time it gets dark. On January 1, Arcturus, herald of spring, rises between midnight and 1 a.m. On March 1 it will be rising four hours earlier, between 8 and 9 p.m. On the Spring Equinox it will be emerging above the horizon at about 7:30 p.m. The rule of thumb is that if a winter constellation is rising at 8 p.m., spring constellations will be coming up at 2 a.m. and the summer ones just appearing in the dawn twilight. When tired of winter and looking forward to spring, stay up late and get a preview of the spring constellations, and remember that spring is coming.
Jupiter continues to dominate the southern sky during the night, like an escaped aircraft landing light. Mars rises around 11 p.m., Saturn around 2 a.m. Venus is a spectacular searchlight in the southwest after sunset. Mercury lies very low in the east before sunrise. The Moon will be full on the 9th.
Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton.