Right Ascension and Declination Help (page 3)

By — McGraw-Hill Professional
Updated on Sep 15, 2011

The Ecliptic

The path that the Sun follows against the background of stars during the year is a slanted celestial circle called the ecliptic . Imagine Earth’s orbit around the Sun; it is an ellipse (not quite a perfect circle, as we will later learn), and it lies in a flat geometric plane. This plane, called the plane of the ecliptic , is tilted by 23.5 degrees relative to the plane defined by Earth’s equator. If the plane of the ecliptic were made visible somehow, it would look like a thin gray line through the heavens that passes through the celestial equator at the equinoxes, reaching a northerly peak at the June solstice and a southerly peak at the December solstice. If you’ve ever been in a planetarium, you’ve seen the ecliptic projected in that artificial sky, complete with RA numbers proceeding from right to left from the vernal equinox.

Suppose that you convert the celestial latitude and longitude coordinate system to a Mercator projection, similar to those distorted maps of the world in which all the parallels and meridians show up as straight lines. The ecliptic would look like a sine wave on such a map, with a peak at +23.5 degrees (the summer solstice), a trough at –23.5 degrees (the winter solstice), and two nodes (one at each equinox). This is shown in Fig. 1-10. From this graph, you can see that the number of hours of daylight, and the course of the Sun across the sky, changes rapidly in March, April, September, and October and slowly in June, July, December, and January. Have you noticed this before and thought it was only your imagination?

Coordinating the Heavens Right Ascension And Declination The Ecliptic

Figure 1-10. The ecliptic (gray line) is the path that the Sun follows in its annual “lap around the heavens.”

Practice problems of this concept can be found at: Coordinating the Heavens Practice Problems

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