(Chapter 3-4) < < prev   next > > (motion of Sun)
The celestial sphere is a model for describing the motion of Sun and stars across our sky.
Of course, we know that Earth revolves around Sun. However, when we go outside at night and watch the stars for a few hours, we see the stars move across the sky, Or when we are outside during the day, we see Sun move across the sky. From our frame of reference, we are not the ones moving, it's Sun and the stars that move. That's the celestial sphere model.
You should know the following characteristics of the celestial sphere model:
summer solstice--when Sun is at its highest declination above the celestial equator. This occurs on approximately June 21. For those of us who live in the northern hemisphere, we call this the first day of summer. For those who live in the southern hemisphere, this is their first day of winter.
autumnal equinox-- at one intersection of the celestial equator and equinox, midway between summer and winter as Sun moves from northern declinations to southern declinations.
winter solstice--when Sun is at its lowest declination below the celestial equator. This occurs on approximately Dec 21. For those of us who live in the northern hemisphere, we call this the first day of winter. For those who live in the southern hemisphere, this is the first day of summer.
vernal equinox-- at one intersection of the celestial equator and equinox, midway between winter and summer as Sun moves from southern declinations to northern declinations.
Watch this 3-D animation of Sun revolving around Earth in 1 year. Each step in the video represents 1 24-hr day. Thus, 365 steps in the video takes Sun around Earth in one year. At each solstice or equinox, the animation will pause so that you can identify the solstice or equinox. A similar simulation is shown below. In this one, you can click on the scene to advance Sun.
You should definitely know the definitions of each of the solstices and equinoxes and be able to identify them by the position of Sun on the celestial sphere.
What coordinates are used to identify a location (such as city) on Earth? You can see the coordinate system in the picture above.
Though the stars in the night sky are at various distances from us, we only need to specify the coordinates of the star on the celestial sphere. Therefore, in this model, all stars are on the surface of the celestial sphere.
For the location of stars on the celestial sphere, we use coordinates similar to latitude and longitude, but they are called right ascension and declination.
Right ascension is the angle around the celestial sphere from the vernal equinox to the star. Right ascension is measured as an angle in units of hours (and minutes and seconds). There are 24 hours (of angle, not time) around the sphere. There are 60 minutes in an hour and 60 seconds in a minute.
Lines of constant right ascension are shown in this image. They measure how far around a star is (going counterclockwise if viewing it from the celestial North Pole) from the vernal equinox. For example, in this picture, Sun is at a right ascension of RA=12 hours.
Sun at summer solstice is at RA=6 hours. What is the right ascension of Sun at winter solstice?
Declination is the angle up from the celestial equator to the star. Declination is measured in the more traditional unit used for angles--degrees. 0 to 90 degrees is above the celestial equator (North Celestial Pole at 90 degrees), and 0 to -90 degrees is below the celestial equator (South Celestial Pole is at -90 degrees).
Positive angles are used for northern declinations (i.e. above the celestial equator). Negative angles are used for southern declinations (i.e. below the celestial equator).
For example, the declination of Sun at the equinoxes is 0 degrees, and the declination of Sun at summer solstice is at 23.5 degrees. Note that the ecliptic never gets above +23.5 degrees declination nor below -23.5 degrees declination.
What is the declination of Sun at wiinter solstice?
This picture shows lines of constant right ascension and lines of constant declination on the celestial sphere. This forms a grid which we can use to identify the location of a star.
Take the very bright star, Sirius, for example, that is being pointed to by the arrow in this picture. Sun is at the autumnal equinox. The vernal equinox is at the left edge, so 0 h right ascension is the white line on the left side of the sphere. If you start at 0, an count the white lines to the right, you can see that Sirius is between 6 h and 7 h right ascension. So it's at RA=6.5 h. But astronomers usually don't express angles like that. Since there are 60 minutes in an hour, the right ascension of Sirius is approximately RA=6 h, 30 min.
Because Sirius is below the equator, its declination is negative. These lines of constant declination are drawn every 10 degrees. Therefore, Sirius' declination is between -10 d (stands for degrees) and -20 d. It looks like it's about -16 d.
Therefore, the coordinates of Sirius are RA=6 h, 30 min and DEC=-16 d.
Here is a 3D model that you can rotate.
You should know the definitions of right ascension and declination and what angles they measure. Also, know where 0 RA and 0 Dec is defined.
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