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;
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;
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.
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.
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'.
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.