| When is the best time to see the Andromeda Galaxy, known also as M31? What about M42, the Great Nebula in Orion? That depends on several factors. First, where you are on the planet Earth, second, what time of year it is, and third, when you like to do your observing. The best time to see anything through our atmosphere is when the sky object is near the meridian, the imaginary line that runs through the North Pole point in the sky, through the zenith (the point directly overhead), and through the South Pole point in the sky (which will be below the horizon for Northern Hemispherians). Let’s examine these three factors individually.
Since the Earth is roughly spherical, and since we have divided it up into roughly 24 time zones to match the 24 hour clock we use, where you are on the Earth is important. You will see a different sky if you were standing on the North Pole than if you were standing on the South Pole. Also, since the Earth rotates in a continuous fashion and the time zones are quantized, this also creates a problem. For example, say you were twenty feet east of the imaginary line which divides two time zones, and you had a partner with whom you were in contact that was forty feet west of you, so that they were twenty feet west of the time zone line. Hence, their clock would measure one hour earlier than yours, so that when a certain star crossed the meridian, they would record it an hour earlier than you did, even though the two of you saw this passage at virtually the same instant. Likewise, if you were on the far eastern edge of a time zone and you had a partner on the far western edge of the same time zone, you would record the meridian passage of a star nearly an hour earlier than your partner. So, it matters where on the Earth you stand. |
