Our Varying View of the Sky

Introduction

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;

• The rotation of the Earth on its axis
  It takes the Earth one day, that is 24 hours, to rotate once. This corresponds to completing a full circle, 360^o, in 24 hours, which is a rate of 360/24 = 15^o/hour.
  
  
• The orbit of the Moon around the Earth
  It takes the Moon approximately one month, actually 29½ days, to complete the cycle from the New Moon phase back to another New Moon. This corresponds to completing a full circle, 360^o, in 29½ days, which is a rate of about 12^o/day.
  

• The orbit of the Earth around the Sun
  It takes the Earth one year, that is 365 days, to orbit once. This corresponds to completing a full circle, 360^o, in 365 days, which is a rate of 360/365 which is very nearly 1^o/day.
  

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;

• The apparent motion during a single night.
• The apparent motion from night to night.

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 15^o/hour from west to east, then from your point of view everything in the sky appears to move from east to west, also at 15^o/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 15^o further to the west. By 11 p.m. it will have moved to a position which is 30^o 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 15^o/hour, but its own orbital motion around the Earth makes it appear to move at a rate of 12^o/day = ½^o/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 14½^o/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.

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 1^o. This alters our view of the sky by the same 1^o. If we combine this angle with our rotation rate (15^o/hour) this corresponds to time difference of 1/15^th 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 12^o 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 12^o to the east. Seven days from now it would be 84^o further to the east, which puts it just above the eastern horizon. In this position it is close to 'Moon Rise'.

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.