Our Varying View of the Sky


There are 3 different motions which affect the way our sky appears to us. Fortunately these happens at markedly different rates, which allows us to treat them separately. It is important to keep these three rates in mind. They are;

Types of motion

We shall now use this to describe how the different astronomical objects appear to move to us. We shall split the description into two parts;

As we do this is it worth bearing in mind that the objects that we shall be looking at are so very far away from us that they are for all intents and purposes stationary, or at least nearly so. The only except is the Moon, being a mere ¼ million miles away. We shall have to talk about the Moon separately.

During a single night

Within the short time frame of a single night, only a few hours, the only one of the above rates to have any appreciable effect is the rotation of the Earth, and even that is only apparent with careful observation. If you image a fixed sky above you, and picture yourself on an Earth turning on its axis at a rate of 15o/hour from west to east, then from your point of view everything in the sky appears to move from east to west, also at 15o/hour.

Motion of a star in the course of an hour

Suppose, then, that you go outside at 9 p.m. and notice a star which is directly to the south of you. At 10 p.m. that same star will now be 15o further to the west. By 11 p.m. it will have moved to a position which is 30o further to the west.

 The exception to this is the Moon. Our rotational motion again makes it appears to move across the sky at a rate of 15o/hour, but its own orbital motion around the Earth makes it appear to move at a rate of 12o/day = ½o/hour in the opposite direction. Putting these together the net motion of the Moon across the sky is from east to west, but at a rate of only 14½o/hour. The difference between this rate and that for all the other objects is barely noticeable, unless it happens to be very close to a bright star or planet. In that case you can see the distance between the Moon and the star or planet noticeably change as the night progresses.

Motion of the Moon

From night to night

Over longer time periods the slower rates accumulate noticeable effects. If it were not for these the night sky would look the same every night, whereas there are actually small changes.

As we said above, each night the Earth moves in its orbit around the Sun, by about 1o. This alters our view of the sky by the same 1o. If we combine this angle with our rotation rate (15o/hour) this corresponds to time difference of 1/15th of an hour, or 4 minutes. Any star, one day later rises 4 minutes earlier. For example, on February 26, 2004 for an observer in Turlock the star Gamma Leo rises at 16:54. The next night (February 27) it rises at 16:50. On February 28 it rises at 16:46 and so on. If you accumulate 4 minutes every night over the period of a year, then the sum is equal to one full revolution of a 24 hour clock. On February 26, 2005 this star again rises at 16:54.

For non stellar objects we judge the position not relative to ourselves, since this would include the effects of our own rotation, but relative to the stars' positions.

As before the major exception to this is the Moon, because it is so close to us. From one night to the next it moves to a position about 12o further to the east. Suppose that at 9 p.m. tonight the Moon is due South of you. Tomorrow at 9 p.m. it would be 12o to the east. Seven days from now it would be 84o further to the east, which puts it just above the eastern horizon. In this position it is close to 'Moon Rise'.

Daily motion of the Moon

The planets are the intermediate case between the Moon and the stars. They are much closer than the stars, so their motion can be detected, by also much further away than the Moon, so that their motion is very slow. From one night to the next the change in their position is negligible, but after a long time this motion can become apparent. How long depends on how close the planet is to us. In the case of Venus, which is very close, after just a few weeks the position of the planet relative to the stars has changed. For a planet which is much further from us, such as Saturn, it can take many weeks, even months, before you notice that the position has changed.

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