Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Can someone explain thoroughly the celestial sphere

  1. Jan 26, 2014 #1
    I am trying to get into amateur astronomy but first I have to understand the sky. Because of the tilt of the earth, there is more than half the surface area of the earth radiated by the sun's rays and therefore those areas which are on the hemisphere of more sunlight have longer days. Also using the model of the sun at the centre and earth revolving round it, the concept of seasons is clear.
    My problem with the celestial sphere concept is I cannot use it to explain everything that I already know about the sun's position on the sky, how the sun changes its position from summer to winter. I am not even sure if I know if the the depiction of the path of the sun on the celestial sphere is the path on a given day or a year. As you can see I am confused and most sites I visit just define stuff without giving reasons or addition information such as how the path of the sun on the celestial sphere is constructed, how the celestial sphere changes throughout the year etc.
    Please explain if you can or direct me to some resource which thoroughly explains the concept, not just a site with definitions.
  2. jcsd
  3. Jan 26, 2014 #2


    User Avatar

    Staff: Mentor

    The tilt merely exposes more or less of the northern and southern hemispheres to sunlight at a time. Ignoring the effects of light scattering and refraction due to the atmosphere, only half the Earth is ever illuminated by the sun. This is easily demonstrated by holding up any spherical object to a light. No matter how you tilt it, the sphere will only ever have half of its surface illuminated.

    I don't have any links on hand, but I can't imagine that a thorough search on google would turn up so little. If no one has any good links I can only suggest to keep looking online.
  4. Jan 26, 2014 #3
    How can you explain position of the sun on the sky during a given day from the projection (ecliptic) of the sun's path across the sky on the celestial sphere (which is the the annual path of the sun)
  5. Jan 26, 2014 #4


    User Avatar
    Science Advisor
    Gold Member

    The earth wobbles on its axis, so the path is slightly different each year. It takes about 26,000 years for the wobble of the earths axis to complete 1 revolution.
  6. Jan 27, 2014 #5


    User Avatar
    Science Advisor

    Perhaps you are confused about the celestial sphere. You should view the celestial sphere as fixed in space as the Earth rotates beneath it. So the Sun and stars rise and set each day as a consequence of the Earth's rotation. The position of the Sun on the celestial sphere changes only slightly during the course of a single day. Since it takes the Sun 365 days (one year) to follow the ecliptic around the celestial sphere, and there are 360 degrees in a circle, the Sun moves a little less than 1 degree eastward along the ecliptic during a single day. The Sun's motion on the celestial sphere is a consequence of the Earth's motion in its orbit around the Sun. Does this help? If not, ask a specific question.
  7. Jan 28, 2014 #6
    Last edited: Jan 28, 2014
  8. Feb 18, 2014 #7
    medwatt, what you are looking for is the analemma.
  9. Feb 19, 2014 #8
    This is probably not the most accurate or scientific, but I see the celestial sphere as pretty much the same as the geocentric view of the sky excluding our solar system as it doesn't fit the scheme.
    Last edited: Feb 19, 2014
  10. Feb 19, 2014 #9


    User Avatar
    Gold Member


    Amateur astronomy is a great area of study, especially because the laboratory is just outside. The two site links that tfr000 provided in post number 6 above are excellent learning tools. I recommend that you try to use them and control all the variables provided.

    Just to be sure you understand the term "celestial sphere" correctly I suggest beginning with this: go outside at night in a "dark sky" location...meaning far away from man-made light pollution. Bring a lounge chair, find an open area, and lay back in your lounge chair. Allow at least 15-20 minutes for your eyes to adjust to the lower light level. Above you 1/2 of the celestial sphere will become visible. The Zenith is directly above you. If you are in the Northern hemisphere you will notice all the stars appear to rotate around Polaris, or the North Star, as our earth rotates. You may find the Milky Way and other constellations. Read more at:
    and http://csep10.phys.utk.edu/astr161/lect/celestial/celestial.html
    and http://astro.wsu.edu/worthey/astro/html/lec-celestial-sph.html

    Now, as for the sun and its connection with our seasons. A simple sundial is a valuable learning tool. Discover all the different types of sundials, then buy or make some simple dials and use them. Consider the “small mirror on the southern windowsill” experiment, for instance. A web search will even direct you to paper/cardboard sundial “how to” construction sites. Experimenting with sundials helps one achieve an intuitive appreciation of how our sun-earth system changes over the year and how the seasons change. Our solar system dynamics is not complicated and can be understood easily.
    see: http://plus.maths.org/content/analemmatic-sundials-how-build-one-and-why-they-work

    This is from Wikipedia:
    “The principles of sundials are understood most easily from the Sun's apparent motion. The Earth rotates on its axis, and revolves in an elliptical orbit around the Sun. An excellent approximation assumes that the Sun revolves around a stationary Earth on the celestial sphere, which rotates every 24 hours about its celestial axis. The celestial axis is the line connecting the celestial poles. Since the celestial axis is aligned with the axis about which the Earth rotates, the angle of the axis with the local horizontal is the local geographical latitude.”

    Finally, visit the website of “Sky and Telescope”, one of the leading monthly magazines for amateur astronomers. It has many great features. Consider a subscription to the magazine; I think it would be well worth the cost.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook