See the seam on the celestial globe halfway between the celestial poles? That’s the celestial equator. See the band of metal circling the celestial globe dividing it in half horizontally? That’s essentially where the ecliptic would be located. In the image below, all the planetary orbits define the plane of the ecliptic.īack to the celestial globe. The Sun is following the path of the ecliptic in the sky (on the Sun, we’d see the Earth following the path of the ecliptic in the sky – it goes both ways!). From here on Earth, we see the Sun move across the sky between sunrise and sunset. They all orbit (roughly) in the same plane. Imagine all of the planets in our solar system orbiting the Sun. Tilted with respect to what? Tilted with respect to the Earth’s orbit around the Sun called the plane of the ecliptic. Why? Because the Earth’s axis is tilted by that amount. The amount of tilt is 23.5 º from vertical. Notice how the north and south pole of the Earth are not oriented vertically but tilted. Look back at the celestial globe picture again. To understand, another complication must be discussed! Where do you place the 00 hour line of right ascension? Once again, it’s arbitrary but a logical place was chosen. Now, just as with longitude on the Earth, there’s a problem when setting up right ascension. So the star, and the declination/right ascension coordinate system ,are both rotating around the Earth (that’s why the star’s declination and right ascension are constant even though we see the stars moving through the sky during the course of the night - the coordinate system is moving too). Then imagine the celestial sphere rotating around the Earth once every a day (there’s a complication here, it’s actually once every 23 hours and 56 minutes, not 24 hours, because the Earth is also going around the Sun as it rotates but don't worry about that here). When looking at the celestial globe, imagine that the Earth is fixed and not moving. In order to understand why that is, we have to complicate things a bit. It always has the same declination and right ascension, not matter what time of day or night or what location on Earth you’re viewing it from. It has a right ascension of 18h 37m 18s (the abbreviations are hours, minutes, and seconds). This time of year, a prominent star in the evening sky is Vega. Vega has a declination of +38 º 47’ 37” (almost 39 º north of the celestial equator). The 24 hours of right ascension increase to the west (clockwise around the celestial globe as viewed from above its North Pole). This differs from longitude on Earth (which has 0-180º E and 0-180º W). ![]() Because the Earth rotates once on its axis in 24 hours, there are 24 hour lines of right ascension (subdivided into minutes and seconds). Right ascension, however, is a bit different. With me so far?ĭeclination at the celestial equator is 0 º, declination at the north celestial pole is +90 º, and declination at the south celestial pole is -90 º (N and S are not used in declination, only +/-). Extend the Earth’s equator outward to define the celestial equator. You’ve now defined the north and south celestial poles. ![]() Now imagine extending the north and south poles of the Earth out to the celestial sphere. Declination, abbreviated with the Greek lowercase “d” or d, is analogous to latitude and right ascension, abbreviated with the Greek lowercase “a” or a, is analogous to longitude. Those are the declination and right ascension lines. Looking at the celestial globe above, you can see lines scribed on it that look like latitude and longitude lines.
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