A short refresher from the last post: we imagined the sky as a giant sphere enclosing the Earth, and call it a Celestial Sphere. We defined four reference points on this giant sphere to help us orient ourselves while looking up – the North Celestial Pole, the South Celestial Pole, the Celestial Equator and the ecliptic. For now, we’ll focus on the first three – the ecliptic only becomes useful when we’re observing planets.
From a single point on Earth, we can only see half the Celestial Sphere at any given moment.
Imagine standing exactly at the North Pole of Earth.
Here the North Celestial Pole will be directly above you, and the Celestial Equator will be where your horizon is, so this becomes the half-sky hemisphere you’ll see:
Now imagine standing at the Equator.
Here the North and South Celestial Poles will be at their respective sides of the horizon, and the Equator will be a line overhead joining East and West.
It’s a really simple idea – whatever latitude you’re standing at on Earth is the Declination that will be overhead. Just picture the equator and the pole closest to you projected onto the Celestial Sphere. These are your Celestial reference points. Draw a straight line through the Celestial Equator from East to West (this doesn’t always go overhead), and take note of where the pole/s are in your hemisphere
To test your understanding, imagine standing at the South Pole and at 45º latitude respectively (see the illustrations below). Try to figure out where the Celestial Equator and each Celestial Pole will be with respect to you.
Try this for your current latitude as well. You should have at least one Celestial Pole and the Celestial Equator in your hemisphere. If you live at the Equator, you’ll get both poles at the horizons!
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