Historical Astronomy: General Observations of the Sky

The Sun

There are a number of somewhat obvious observations of the motion of the sun:

• The sun rises in the east and sets in the west.
• The sun follows an arc across the sky.
• In the winter, there is less daylight than in the summer, and the sun does not get as high in the sky as it does in the summer.
• the seasons are not the same length.
• The sun rises and sets at different places along the horizon, depending on the season. See picture below.
• Every now and then, the moon gets in front of the sun, which we call a solar eclipse.
• This is not so obvious: The sun follows the same path along the celestial sphere, taking 1 year to go around once and therefore traveling about 1 a day. The path of the sun mapped on the celestial sphere is called the ecliptic. Obviously one can only figure this out paying a lot of attention to the stars and remembering what happens.

The picture below shows sunrises at the solstices and an equinox in a town in Greece. This is taken from NASA's Astronomy Picture of the Day. On the summer solstice, the sun will rise at its most northern position on the horizon, and at local noon, the sun will be the highest in the sky that it is all year.

The motion of the sun in the arctic or antarctic makes for awesone time-lapse video, and the internet provides all. YouTube has all sorts of videos of arctic (and antarctic) sunsets and things: the arctic during the summer and the winter solstice in Alaska. If the links failed, just search on north pole sun and you'll get lots of videos, along with posts from nutjobs who think the earth is flat, has holes in it and that NASA fakes everything. That's why I teach, I suppose.

To help visualize all this, there is an excellent sun motions demonstrator at the University of Nebraska - Lincoln.

The Moon

There are a number of somewhat obvious observations of the motion of the moon.

• The moon rises in the east and sets in the west, just like the sun.
• The moon follows an arc across the sky.
• The moon changes appearance from day to day. It follows a regular pattern, going through all its phases in 29.5 days. (This is its synodic period.)
• The moon takes about 27.3 days to return to the same spot in the celestial sphere. (This is its sidereal period.)
• The moon rises about 1 hour later every day.
• The moon moves about 12 along the celestial sphere each day.
• There are periodic lunar eclipses.

UNL also has a good lunar phase simulator.

The Stars

While there are a number of obvious things going on with the stars, one has to pay attention becuase the changes are so gradual.

• The stars rise in the east and set in the west on a nightly basis.
• The stars keep their same relative positions while they move.
• While there are some stars that are always visible, there are some stars that are only visible during certain seasons.

To help visualize the rotation of the celestial sphere, there is an excellent rotating sky explorere at the University of Nebraska - Lincoln. This just shows the effect of the rotation of the earth and doesn't include seasonal changes.

The Planets

One has to pay attention to the stars to notice the planets. To the naked eye, they appear as bright stars. One also has to be very familiar with the stars and constellations to notice that there are 5 stars that are slowly moving. These are called the planets. The easily visible ones are Mercury, Venus, Mars, Jupiter and Saturn. (Mercury is actually tough to see - one has to get it just before dawn or just after sunset.) It turns out that Uranus is also visible to the naked eye, but it is very faint, moves very slowly and is very hard to notice.

• While they appear like really bright stars, their brightness does not flicker like normal stars.
• They slowly move against the celestial sphere, traveling east along the ecliptic.
• They all lie close to the ecliptic, within a band about 10 wide.
• Periodically, the planets travel backwards (west) along the ecliptic for a few weeks. This motion is called retrograde motion
• The brightness of the planets, especially Mars and Venus, changes a lot. The planets are always brightest when they are in retrograde motion.
• Venus and Mercury are never that far away from the sun. They can only appear in the eastern sky in the morning before dawn and in the western sky just after susnet.

The picture below shows a series of pictures of Mars in retrograde motion. There are several days taken between each photo, and it nicely shows Mars do a loop-the-loop against the "fixed" stars. The little streak above Mars is actually Uranus. Photo taken from http://antwrp.gsfc.nasa.gov/apod/ap031216.html

Other Things in the Sky

Looking in the sky, there are a number of other things going on.

• On random, and unpredictable times, there appear very fuzzy stars with "tails" attached to them. These fuzzy stars are visible for a few weeks or months, move across the celestial sphere, getting brighter and then fading away. These are comets.
• Also at random unpredictable times, new stars suddenly appear, sometimes very bright, but fade from view after only a few weeks or months. These look like normal stars, and do not move along the celestial sphere.
• Periodically, there are shooting stars that streak across the sky.
• There are also clouds and rainbows and such in the sky, but they are clearly things that are really close to the surface of the earth.

Celestial Sphere

The picture below shows the key parts of the celestial sphere. We imagine a large sphere centered on the Earth that contains all the stars. The equator of the Earth projects out and is the equator of the celestial sphere, and the axis of the Earth's rotation is the axis of the celestial sphere. (The North Star happens to be pretty close to the North Celestial Pole.) The ecliptic is tilted 23.5 from the celestial equator. The two equinoxes are the times of the year when the sun crosses the celestial equator. (In the diagram, only one equinox is visible, marked "b.") The solstices are the two times that the sun is farthest from the celestial equator, marked "a' and "c" in the diagram. The Zodiac is the group of 12 constellations that are equally spaced around the ecliptic.

Links for More Information:
Nice summary of most of this information, with a lot of good diagrams: http://astro.wsu.edu/worthey/astro/html/lec-celestial-sph.html

last updated 3/5/16 by david mcclung, copyright 2003. All rights reserved.