Our planet also changes positions. The Earth will move from one side to the other in six months. This is an apparent change in position for some stars that could be noticed. Parallax can be used to measure the distance between these stars. Here are some . ways to measure stellar distances . )
Yes, constellations can change, but not as much.
Finding Your Longitude
Here’s how to find your longitude with a clock and a star chart. Let’s begin with the star chart. Let’s say there is a star in that chart that is directly above a Greenwich point at 4 AM local time. We will call this Greenwich Mean Time. I didn’t choose Greenwich randomly. The Royal Observatory Greenwich runs through the prime meridian. It is the line that runs parallel to the . This is useful for measuring. )
Now, let’s pretend that you are somewhere else and are trying to find your location by using the same star. You will need to know what time it is when that star appears directly overhead at your location. The clock is also there.
Checking the time reveals that, where you are, that star appears directly overhead at 1 am, instead of 4 am–three hours earlier than Greenwich. This means that you are three hours west of Geenwich in 24. If you want to convert that to degrees, it would be (3/24) x 360 = 45 degrees. This would place you on the longitude line running through Greenland, Brazil. You can make things more complicated because there is no star overhead. But you get the idea. )
Next, if you are in the northern hemisphere, you can use the North Star to calculate your latitude and determine your exact