When becoming a budging astronomer, even as a hobbyist, it’s important to understand these key concepts and terms when it comes to the sky. We will start with easy concepts before we progress into more difficult ones.
Knowing Your Sky – Simple Concepts
Going outside, you can get to know your key points. You of course have your observing location, but it’s so much more than that.
Directly above you, the highest point in the sky, it’s called the Zenith. The line where the ground meets the sky is called the horizon. Assuming your sky is clear and dark enough, if an object in the sky is not visible, it’s either too close to the Sun’s glare and not visible at night, it has already set below the horizon, or it has not risen yet.
Ideally you want your observing location to be an open sky, but a lot of people have to sacrifice open sky because of hills, trees, or adjacent buildings.
Rising and Setting
You have noticed that during the day, the Sun is not always in the same position, and neither is the lighting – some hours it’s shining into your window, other hours it isn’t.
You’ve seen the bright moon in different positions, sometimes being low, rising behind the mountains, or you’ve seen it high up in the sky.
You have noticed a bright star, or planet. You may have even gotten a telescope on the object, and then keep the telescope still. But when you go outside to look at it again, you notice the object isn’t showing in your telescope anymore, and with the naked eye it’s in a different spot in the sky.
That is because everything in the sky moves! It rises and sets in an east to west movement with the Earth’s rotation. Objects are rising and setting even during the daytime, which factors in how long you’ll be able to see it at night, if at all.
When any object reaches the midpoint, or the highest it will be in the sky, we say it’s crossing the meridian line. When our Sun is doing it during the daytime, we call it noon.
Get To Know A Few Constellations and Bright Stars
There are a total of 88 constellations in the sky. A beginner astronomer only needs to know a few of them, particularly the brighter ones. Don’t worry about the dimmer star patterns just yet.
With the sky on any given night, the most important constellations are those that aid in your cardinal directions. Ursa Major and Cassiopeia are constellations that are close to the north celestial pole, visible nearly all night each year (albeit in different positions), and never set when viewed from mid northern latitudes.
This website has detailed articles on the key constellations for each season visible in the northern hemisphere. You can find it on “The Sky” Page.
Know Your Cardinal Points
In your general observing area, it’s important to know the cardinal points, that way you know the direction you need to face.
You are taught in grade school that everything in the sky, especially the Sun and Moon, rise in the east and sets in the west. While there are variances as the seasons change (which we’ll get to), this is a good general rule. From the northern hemisphere, when the Sun reaches local noon, or when the Moon crosses the meridian line, it’s directly south.
You can also use your local landscape markers to help too. For example, most residents living in or near Los Angeles have high mountains to the north, valleys and deserts to the east, with the beaches to the west and south.
Finding True North (or True South)
Most compasses point to magnetic north, not necessarily true north, and depending on your location, there can be quite a gap between the two.
In the Northern Hemisphere, Polaris’ close proximity to the north celestial pole makes it appear not to move due to Earth’s rotation, pretty much always being in the same position in the sky! That is why it’s used as a navigational tool.
In a dark sky, Polaris can easily be seen as the bottom of Ursa Minor (The Little Dipper). However, from a city sky, while Polaris is still bright enough, the rest of Ursa Minor is too dim. So in that case you use either the Big Dipper in the spring and summer months, or Cassiopeia in the fall and winter months to help find it.
The more north in latitude you are, the higher Polaris is in the sky, and vice versa. Once you cross the equator, Polaris is no longer visible!
So that means you need to find a “South Star,” right? It’s not that easy. Currently, there is no star near the south celestial pole that is bright enough to be used as a navigational star. So the following image is the common method used.
Over the southern hemisphere, constellations like Crux are circumpolar when viewed from mid southern latitudes over the southern horizon. The two “pointers” as diagrammed are α Centauri and β Centauri from Centaurus. Archernar is the southern end of the Eridanus constellation.
Wait… How Could People Tell the Planets Apart From Other Stars?
The planets do appear as bright stars in the sky – well, the naked eye planets do. Venus, Jupiter, and sometimes Mars are brighter than the brightest stars, so they can easily be identified. But Saturn and Mars are usually as bright as the other stars, so they can be mistaken.
But even ancient astronomers could see that while the constellations appeared fixed due to Earth’s rotation, these special stars appeared to change positions over time! Some changed positions quickly, while others were slower. But clearly there was something unusual about these stars.
he ancient Greeks called them πλάνητες ἀστέρες (planētes asteres, “wandering stars”) or simply πλανῆται (planētai, “wanderers”). That’s where the word planet comes from!