Part 1 - Zenith, The North Celestial Pole and Circumpolar Constellations
In this first part of Module 3, you will learn the most important parts of the night sky, those which never change. If you remember these you will always recognize a part of the sky, no matter what time of year or day you are looking at it.
Things That Never Change
There is so much variety in the night sky, by hour, season and year, that finding your way around it can feel overwhelming.
Reading that you need to find 'Star X' 18° above the western horizon an hour after sunset can feel like you're reading Dutch, for all the sense it makes.
When magazines and websites (including my own) say things like 'it lies very close to Aldebaran in Taurus', they are actually trying to be helpful. But, if you're brand new to astronomy and have no idea what (or where) Aldebaran or Taurus are, then it's not helpful at all.
To give you a grounded starting point in finding your way around, I'm going to show you some features of the night sky that never change, no matter what time of day or year it is. These unchanging features are what we use as a starting point to finding every other part of the night sky.
The unchanging parts of the sky we'll look at below are: the Cardinal Points; the Zenith; the North Celestial Pole, and: Circumpolar Constellations.
The Cardinal Points
Before we into the sky itself, let's begin with the cardinal points of a compass: north, south, east and west.
Wherever you are observing from, it helps to know which way is which. Use the compass on your cell phone (or a proper compass) to help you.
Just like the sun, all the stars, deep space objects and planets move across the sky from east to west and are at their highest when due south. This is useful to know so that you see objects at their best (highest) during your observing sessions and don't leave it too late to see them, so they've set below the horizon.
You can see me demonstrate this east to west movement in the video, below.
Space appears to us like a dome stretching from the horizon to above our head. That point directly overhead is known as the zenith and relates to you as an observer.
No matter what time of day you're looking, or where you are standing, the point directly overhead is the Zenith. You'll see it marked on a number of the Sky Safari 5 videos in this course as a green cross. I use it as a handy reference point when planning what to look at.
One zenith oddity to consider when you look at any star map is that there is nothing 'above' the zenith, even though the maps make it look like there is. For example, if you have north in front of you on your screen / map but see an object 'above' the zenith, it is actually in the south!
Now, this may not make sense as you read it, so watch the next video which explains it more effectively.
The North Celestial Pole
Imagine for a moment that you are standing at the top of the world. Beneath your feet is the North Pole and, above your head is...
The North Celestial Pole.
The north celestial pole (NCP) is the stationary point in the northern hemisphere sky (there is also a south celestial pole above the south pole) around which all the planets and stars - including our sun - appear to rotate.
In the picture below, the NCP is in the center of the small circle towards the top right of the picture. The north celestial pole does not move from this point but all the stars in the sky rotate around it.
Note how some will never drop below the horizon whilst others do. The stars that don't set below the horizon are called circumpolar - and there's more on that, and why it's useful to backyard astronomers, below.
Stars Circling the North Celestial Pole
The tricky thing with the NCP is that although it never moves in your sky, its height above the horizon depends on how far north or south you are. As I said at the start of this section, if you are standing at the north pole the north celestial pole is at the zenith, i.e. it is directly overhead.
Move further south though and the NCP begins to sink towards the horizon, but is always found by facing due north.
In Anchorage, Alaska, for example, the NCP is about 60° above the due north horizon. At the Canadian border it's 49° above the northern horizon, in Kansas City, Missouri, you'll find it 40° above the horizon and in Key West, Florida, you'll see it just 24° above the northern horizon.
Keep travelling south to the equator and the north celestial pole is on the horizon. South of the equator, i.e. in the southern hemisphere, the NCP is below the horizon and we can now see the south celestial pole rising due south.
The easiest way to find the NCP for yourself is to locate the star Polaris, which is in the tail of the Little Dipper (Ursa Minor). For all practical purposes Polaris marks the point in the sky which is the north celestial pole.
Further down the page, in the Circumpolar Constellations section, you can see exactly what Ursa Minor looks like, how to find it and which star within it is Polaris.
You probably already know that a constellation is an area of sky named after the shapes made by the brightest stars. The most well recognized constellations are the Big Dipper and Orion.
The Big Dipper and Orion Constellations
But, did you know that there are 36 constellations in the northern hemisphere?
Some of them are small, some are faint and some spend barely any time above the horizon. And the thought of trying to learn 36 constellations so you can find your way around the night sky is a daunting prospect.
Fortunately, you really don't need to!
Instead, it is easier and more useful to learn the small number of constellations which are in the sky all of the time: the Circumpolar Constellations.
When I say they are in the sky all the time, I mean all of the time! No matter the hour of day, day of the month, or month of the year, these few constellations never set below the horizon. For this reason, these are the markers all of us as astronomers should learn to help us find our way around the rest of the sky.
As you saw above, in the north circular pole section, the word 'circumpolar' means they circle the north celestial pole without ever setting below the horizon. The NCP is higher in the sky the further north you go, and the higher the NCP is in the sky, the more room there is between it and the horizon. This means people living further north see more circumpolar constellations than those of us living further south. At its extreme, every constellation is circumpolar at the north pole because the NCP is overhead.
There are five constellations which are circumpolar in all states except Florida and Hawaii:
- Ursa Minor (The Little Dipper)
- Draco (not Hawaii)
- Cassiopeia (not Florida and Hawaii)
Further north there are more circumpolar constellations.
As you'll see in the video below, Ursa Major (the Big Dipper) is circumpolar in states from around the mid-US (e.g. Nevada, Utah, Kansas, etc) northward. Up at the Canadian border as many as 10 constellations are circumpolar.
To find out exactly which constellations are circumpolar for you, see my detailed article on Love the Night Sky. There's a sortable table on the page, just find your state and discover your circumpolar constellations.
See the next two videos which show you more about circumpolar constellations. The first one shows you how they work, the second shows how you can set Sky Safari 5 to show you the circumpolar stars in your own sky.
After the videos, in the final section of this subject, I'll take you through the five circumpolar constellations that practically every US citizen can see... plus a bonus 6th!
How Circumpolar Constellations Work
This is the link to my circumpolar constellations article I mentioned in the video above. Use it to see which constellations are circumpolar in your state.
Staying with Circumpolar stars, I show you in the next video how to set up Sky Safari so that you can see the circumpolar constellations for wherever you carry out your backyard astronomy.
How to Set Up Sky Safari to See Your Circumpolar Constellations
The Five Circumpolar Constellations & The Big Dipper
I'll begin with Ursa Minor, which is closest to the NCP, then work further away to Cassiopeia. I'm going to finish with the Big Dipper (Ursa Major) which is circumpolar for around half of the US. It is so iconic, recognizable and useful for all astronomers that it should be in your list of 'recognized constellations' even if it is not circumpolar from your state.
The following screen grabs from Sky Safari 5 show the circumpolar constellations (with and without the imaginary lines of the constellations) at different levels of sky darkness.
These first two are for light polluted skies, and show all the brightest stars visible in the circumpolar constellations, down to magnitude 4.
Magnitude 4 - With Constellation Lines
Magnitude 4 - Without Constellation Lines
These next two images reflect what you should be able to see under a darker sky, but one where light pollution still blocks out fainter stars. They each show circumpolar stars to magnitude 5.
Magnitude 5 - With Constellation Lines
Magnitude 5 - Without Constellation Lines
Finally, these last two images show the stars you can see with the naked eye in and around the circumpolar constellations at a dark site. The stars in these images go down to magnitude 6.
Magnitude 6 - With Constellation Lines
Magnitude 6 - Without Constellation Lines
For the last part of this section, I'll give you a brief overview of the circumpolar constellations. Please use the images above (or the links to their Wikipedia pages, below) to see what each one looks like.
Ursa Minor (the Little Dipper)
This small constellation holds the honor of being at the centre of our northern sky. It is a fainter and smaller than its Big Dipper brother, but has a very similar shape.
The brightest star in the constellation is Polaris, which, as you read above, is effectively the location of the north celestial pole.
You'll find Polaris at the end of the dipper's handle. It shines at magnitude 2.0 and so is visible even in heavily light polluted, urban skies. Being within half a degree of the north celestial pole, Polaris is always be found above the due north horizon (which is why it's been used for centuries by sailors to navigate at night).
Use your compass to point the right way and - depending how far north or south you are - find it between 25° and 50° above the horizon. The next section shows you an easy way to estimate this distance using just your hands.
Cepheus is the king married to Cassiopeia (see below) in Greek mythology. The bulk of the constellation looks a little like a small child's drawing of a house, which you can best see on the 'Magnitude 6 - With Constellation Lines' picture, above. There is a 'square' of bright stars which is topped by a triangle which roughly points to Polaris.
Evening viewing is best in the second half of the year when the constellation is higher than the NCP. In the first half of the year Cepheus sits between the NCP and the horizon, making it a poorer spectacle.
This is supposed to represent a giraffe.
I can kind of see it, but it's a stretch! What do you think?
This constellation is rarely useful. Although it is circumpolar, the brightest star within it is magnitude 4, which makes the whole constellation largely unhelpful, especially in a light-polluted sky.
If you do want to find it, best times are over winter when it is highest in the evening sky.
Draco the dragon is a great, winding constellation which is best picked out in the northern sky through its winding between the Big and Little Dippers ending at the box of four stars forming the dragon's 'head'. Again, the best view of this is on the 'Magnitude 6 - With Constellation Lines' picture, above. I use these to find Hercules, as that is where they seem to be looking.
The Queen of Ethiopia and wife of Cepheus (above). I think this is one of the most distinctive constellations in the night sky. The bright 'W' (or 'M', if you prefer) is impossible to mistake.
She looks at her best in the fall and winter skies as her five brightest stars (four of which are brighter than magnitude 3) shine opposite the only constellation which is easier to find than she is: the Big Dipper.
Ursa Major (The Big Dipper)
If you are in southern states Ursa Major will not be circumpolar, but it is for mid and northern states and is so easy to identify that I feel it earns a place in this section.
Ursa Major is the Great Bear constellation. It includes around 18 bright stars and does - with imagination - resemble a bear. However, most of us know and recognize a smaller part of the constellation called the Big Dipper.
The Big Dipper is actually an asterism, which is a group of stars that look like something and have their own name, but which are not a constellation in their own right. The Big Dipper looks like a dipping ladle, saucepan or plough (as it is known in the UK).
It's best seen in spring and summer evenings and is a very handy pointer to the stars Polaris and Arcturus, as you can see in the picture below.
Using The Big Dipper to Find Polaris and Arcturus
Well done, you made it to the end of this section on the Zenith, North Celestial Pole and Circumpolar Constellations. If you commit just these three things to memory you will be further in front than many backyard amateurs.
Next, it's time to teach you a really simple technique for using just your hands to measure night sky distances.