The Seasons There would be no seasons if the Earth were not tilted on it’s axis!  Nick Devereux 2006

The Sun appears to circle the Earth, (only an illusion), along a trajectory that is called the ecliptic.  Nick Devereux 2006

Equinoxes and Solstices The Sun reaches the zenith at noon on the equator twice a year, once in March (Spring Equinox) and six months later in September (Autumnal Equinox). The Sun reaches the zenith at noon on the tropics twice a year, once in December (Winter Solstice, Tropic of Capricorn) and once in June (Summer Solstice, Tropic of Cancer).  Nick Devereux 2006

Sun is perpendicular to the local horizon at Tropic of Capricorn

Sun is perpendicular to the local horizon at Tropic of Cancer  Nick Devereux 2006

As a result of the earths tilt, the Suns elevation above the horizon changes constantly through out the year which leads to seasons.  Nick Devereux 2006

For the same reason, the night time sky changes also. If you go out at the same time each night different stars and constellations will be overhead and they constantly change throughout the year

 Nick Devereux 2006

The Earth moves through an angle of 360 degress in 365 days relative to the Sun, which works out to be 0.98 degrees/day. Now, the Earth also spins on it’s axis 360 degrees in 24 hours which works out to be 15 degrees per hour, or 15 degrees/60 min = 0.25 degrees/minute. So, if you want to see the stars in exactly the same place in the sky as the night before, you have to go out 0.98/0.25 = 3.9 which is ~ 4 minutes earlier than the previous night!  Nick Devereux 2006

The Celestial Co-ordinate System The Celestial co-ordinate system is analogous to latitude and longitude. Declination is analogous latitude, as both are measured with respect to the equator. The celestial equivalent of Longitude is a bit more complicated because the Earth is turning on it’s axis, and moving about the Sun, so the projection of the Earth’s longitude system on the night sky is constantly moving. Astronomers have thus adopted a specific location on the sky, known as the “First Point in Aries”, that is actually the intersection of the ecliptic and the celestial equator, (where the Sun is on the Spring equinox), to mark the zero point of the celestial longitude system, called Right Ascension.  Nick Devereux 2006

Sidereal Time Sidereal Time is just the hour of RA that is passing overhead. So, if the vernal equinox (0 hrs RA) is crossing the meridian, the sidereal time is 0hrs, and 1 hour later, it will be 1 hr and so on. Here’s another way to think about it; The RA of an object is the time between the vernal equinox crossing the observers meridian and the object doing the same, but this requires some knowledge of where the vernal equinox is, which you can figure out since you know that the Sun is at the vernal equinox (0 hrs) on March 21st, so 0 hours RA is on the meridian at noon, which means that 12 hours RA is on the meridian at midnight. You can figure everything else out from that and knowing that the Earth advances 1 o or 4 min each day, or 2 hrs each month.  Nick Devereux 2006

We can also calculate the elevation of the Sun at each of the 4 cardinal dates for any latitude on Earth….

 Nick Devereux 2006