The Celestial Sphere and Observation Planning

If I want to view the star Tau Ceti, I need to know where it will be at what time of the night so I can find it. With just a little bit of knowledge about Tau Ceti's location as well as my own on Earth, I know everything I need to about how to find Tau Ceti.

Let's say I'm observing from Palomar Observatory, which has a latitude of 33º 21' 21'' N and a longitude of 116º 51' 50" W. If I only want to view Tau Ceti when it's on my meridian (the line that runs from north to south directly overhead through the zenith), then it's much easier to figure out where it will be in the sky at what time.

Since an object's right ascension and declination don't change over the course of the year, Tau Ceti's position on the meridian will be the same every night, and the only difference will be the time that it is on the meridian. In order to figure out Tau Ceti's elevation on meridian each night, I need to know its declination and my observation latitude. Tau Ceti's right ascension is 1:4:04.0829 and declination is -15º 56' 14.928". A negative declination means that, from the equator, Tau Ceti will be about 16º below the zenith when it's on the meridian. But I'm not observing from the equator, so I have to subtract my latitude. 33 + 16 = 49, and since I'm 33º north of the equator, it makes Tau Ceti lower in the sky, so its position is -49º (see first image, "Finding Elevation of Tau Ceti").

Now that I've determined the location of Tau Ceti when it's on my meridian, I just need to determine the time of the night that Tau Ceti will be on my meridian so I know when to look for it. The time system LST (local sidereal time) is measured by the stars and thus Tau Ceti will be on my meridian at the same time LST each night all year. But, since the Earth is orbiting the sun as it rotates, LST does not exactly match up with time on earth. One day in LST has an extra four minutes, which means that each month LST has an extra two hours than a month in UT (universal time, measured by days on Earth). LST has its zero point at the vernal equinox, at noon on March 21st, so LST and UT line up at this time. Each month later, LST is an additional two hours later at UT's noon. For example, an LST of 0:00 falls at a UT of 2:00 pm in late April, because April is a month later than March. The graph of Tau Ceti's elevation on the meridian shows the location of Tau Ceti in the sky as well as the time at which it is on the meridian each month of the year (see below).

Now I have everything I need to determine when to observe Tau Ceti from Palomar Observatory!



Primary author: Cassi Lochhaas
Contributors: Joanna Robaszewski, Juliette Becker