The Big Dipper: the Northern Signpost
The Big Dipper is one of the most easily recognisable star patterns in the night sky. Also known as The Plough in the UK, it is a great starting point to explore and learn nearby constellations.
This astronomy essentials post will introduce you to The Big Dipper and how to find it in the night sky. I will then show you how to use The Big Dipper to find the North Star and 6 other important constellations.
The Big Dipper, by Tobias Barz CC BY-ND-2.0
1. Getting to know The Big Dipper
The Big Dipper is not a constellation, but a group of easy-to-recognise stars known as an asterism. It makes up part of the large constellation Ursa Major or Great Bear.
Likened to a saucepan The Big Dipper is made up of seven stars, shown in the star chart below. Six out of the seven stars are bright second-magnitude stars of roughly equal brightness, making them stand out against other nearby stars.
The stars are also evenly spaced apart forming the Big Dipper’s distinctive shape, which helps to make this an easy pattern to identify.
The four stars: Megrez, Phecda, Merak and Dubhe outline the ‘bowl’ section, and the remaining three stars: Alkaid, Mizar and Alioth trace out the ‘handle’.
More details of the seven stars that make up The Big Dipper are listed below, starting with the brightest, Alioth.
| Name | Apparent magnitude | Distance (light years) |
|---|---|---|
|
Alioth |
1.77 |
82.6 |
|
Dubhe |
1.79 |
123.0 |
|
Alkaid |
1.86 |
103.9 |
|
Mizar |
2.04 |
82.9 |
|
Merak |
2.37 |
79.7 |
|
Phecda |
2.44 |
83.2 |
|
Megrez |
3.31 |
80.5 |
| Name | Apparent magnitude | Distance (light years) |
|---|---|---|
|
Alioth |
1.77 |
82.6 |
|
Dubhe |
1.79 |
123.0 |
|
Alkaid |
1.86 |
103.9 |
|
Mizar |
2.04 |
82.9 |
|
Merak |
2.37 |
79.7 |
|
Phecda |
2.44 |
83.2 |
|
Megrez |
3.31 |
80.5 |
The most interesting star you should make note of is Mizar, the second star from the end of the handle. It is both a line of sight ‘double star’ and a true binary star system.
Nestling close to Mizar you can just about make out a fainter star, known as Alcor. Resolving this line of sight ‘double star’ is a good test of your naked eye vision.
If you were to point a small telescope at Mizar you would see that it is made up of two stars, known as Mizar A and B. It is one of the first double-star systems that you are likely to observe through a telescope.
2. Can I see The Big Dipper?
The Big Dipper is one of the most famous asterisms in the northern sky. If you live north of latitude 41°N then The Big Dipper will be circumpolar and you will be able to look up every night of the year and find this familiar group of stars.
Further south it will become more difficult to view, sinking closer to your northern horizon. If you are located south of latitude 41°S then, unfortunately, none of its stars will be visible to you at any time of the year.
If you can see The Big Dipper then it is best viewed in the Spring (April to June) when it will be highest in the sky.
3. Where is The Big Dipper tonight?
The position and orientation of The Big Dipper in the night sky change depending on the time of year and the time of night that you view it.
To help you locate it more easily you can use the following illustration.
The Big Dipper position throughout the year and throughout the night, by Mike Soulby stellardiscovery.com
It shows how the position of The Big Dipper changes throughout the night due to the rotation of the Earth on its axis. But it also shows how its position slowly changes throughout the year.
This is best shown on a planisphere, which allows you to set the position of The Big Dipper and instantly see all of the times and dates that it appears at that position in the sky.
A helpful phrase to remember is “Spring up, Fall down” representing the season and the position of the big dipper in the evening sky.
4. Finding the North Star and Ursa Minor (The Little Dipper)
Probably the most helpful star you can find using The Big Dipper is the North Star, also known as Polaris.
The North Star is located less than one degree from the north celestial pole. If you were to stand at the North Pole and look directly above your head then you would see Polaris.
As Polaris is almost directly in line with the Earth’s axis of rotation it remains almost entirely motionless, whereas all the other stars will appear to rotate around it. You can see this in long-exposure star trail photos like the one I took below.
Stationary Polaris by Mike Soulby © stellardiscovery.com
This means that its position is fixed in the sky, always pointing towards true north.
To find the pole star using The Big Dipper you should first find the two stars at the end of the bowl section, named Merak and Dubhe. Then, if you draw and extend a line from these two pointer stars you will find Polaris.
Polaris is a second-magnitude star and the brightest star in the constellation Ursa Minor, or Little Bear.
Ursa Minor is also known as The Little Dipper due to it having a similar shape to The Big Dipper. It also contains seven stars, however, it is smaller and fainter making it a little more tricky to make out.
Polaris marks the end of the ‘handle section’ as shown in the chart above. Identifying The Little Dipper coming out from Polaris is a good way to confirm that you have indeed found the North Star.
How to find Cassiopeia
Another famous northern constellation is Cassiopeia, the Queen. It has a distinctive ‘M’ or ‘W’ shape and is made up of bright stars, so it is likely that you can spot this constellation without too many difficulties.
To help you find Cassiopeia first find the North Star, as described in the previous section.
By using the North Star and the final star in the handle section of The Big Dipper, Alkaid, you can draw an imaginary line across the sky to reach Cassiopeia.
Cassiopeia is made up of 5 bright main stars of second and third magnitude arranged in a very easy-to-recognise ‘M’ or ‘W’ shape.
It is best seen in the Fall (autumn), from September to November, when it will be highest in the sky. For observers north of about 34°N Cassiopeia is circumpolar and visible year-round.
Cassiopeia is a great constellation to scan with binoculars as part of the Milky Way passes through it. It has rich star fields and some open clusters for you to discover.
How to find Boötes
Boötes, the Herdsman, is a great spring constellation best viewed from April to June. It also contains the fourth brightest star in the sky, Arcturus.
The best way to find Boötes is to first locate the bright orange star Arcturus. You can do this by using the handle section of The Big Dipper. If you continue the curve of the handle you will reach Arcturus and the constellation Boötes.
A common phrase to help remind you is, “Follow the arc to Arcturus”.
Boötes extends northwards from Arcturus in a kite or diamond shape.
Boötes contains a fascinating binary star called Izar. The contrasting colours of its two stars make it a popular telescope target.
Izar’s primary star is an orange giant star with an apparent magnitude of 2.37, whilst its companion star is fainter a blue/white main sequence star with an apparent magnitude of 5.12.
The angular separation of these stars is just 2.85 arc seconds so you will need a telescope with a moderate aperture of about 100 mm to resolve them clearly.
How to find Auriga
Auriga, the Charioteer, is best viewed from December to February when it will be highest in the sky. It is best viewed from the Northern Hemisphere and contains the sixth brightest star in the sky, Capella.
To find Auriga you can use the top two stars of the ‘bowl’ section of The Big Dipper, named Megrez and Dubhe. You should draw a line between these two stars and then extend it until you reach the bright yellow star, Capella. The chart below will help you visualise this.
A good way to confirm you have reached Auriga is to look for a small asterism of three stars, called ‘The Kids’, located close to Capella.
Auriga doesn’t look much like a Charioteer; it has a roughly pentagonal shape. Besides Capella, it has one other second-magnitude star and several fainter naked-eye stars.
Auriga is worth identifying as it contains three noteworthy Messier objects: M36, M37, and M38. All are open clusters that are visible in binoculars or a small telescope.
The richest of these is M37, it has an apparent magnitude of 6.2, so even in dark skies is a difficult naked-eye object. Through binoculars, it will appear as a hazy patch, but when viewed through a small telescope you will be able to resolve dozens of closely spaced stars.
How to find Gemini
Gemini is one of the 12 constellations of the Zodiac. It is also known as The Twins as it depicts the twins Castor and Pollux from Greek Mythology.
The two brightest stars in Gemini are named after the twins. Pollux is the brightest with an apparent magnitude of 1.14, closely followed by Castor at 1.58.
Gemini is best viewed from January through to March, but can still be seen in the early evening sky well into April and May.
To find Gemini you can use the stars Megrez and Merak from the ‘bowl’ section of The Big Dipper. You should draw a line through these two stars and continue until you reach the two stars Castor and Pollux.
These two bright stars are located close to each other in the night sky, which helps to make finding and identifying Gemini more straightforward.
Gemini extends out from Castor and Pollux as two chains of stars; one for each twin. Most of these stars are only third or fourth magnitude, with the exception of the brighter second-magnitude star, Alhena.
You should try to locate the open cluster M35, located just off the end of Gemini. This cluster has an apparent magnitude of 5.1 so is visible to the naked eye in dark skies. Binoculars will reveal some of the cluster’s brightest stars, however, a telescope will reveal a rich star field with dozens of stars.
How to find Leo
Leo, the Lion, is another great spring constellation. You should look out for Leo around midnight in March or shortly after nightfall in May when it will be highest in the sky.
To find Leo you can use the stars Megrez and Phecda from the ‘bowl’ section of The Big Dipper. By following this imaginary line you will reach the bright first-magnitude star, Regulus, in Leo.
Moving up from Regulus you will notice a distinctive curved shape made up from other bright stars. This is an asterism within Leo known as ‘The Sickle’ and it resembles a back-to-front question mark symbol.
Leo is one of the few constellations that resembles what it is meant to depict. It is not too difficult to imagine a sitting lion using the stars that mark out the constellation.
Leo hosts the famous Leonids meteor shower every November. The radiant is located within the curve of the Sickle asterism. The Leonids meteors are caused as the Earth passes through cosmic debris left behind by the comet Tempel-Tuttle.
Every 33 years the Leonids puts on a particularly spectacular show, where hundreds or even thousands of meteors can be spotted every hour! The next Leonid meteor storm is expected in 2031.
Last Words
As you can see The Big Dipper is a really useful asterism for night sky navigation. Not only can you use it to find the North Star, but you can also easily jump to several other nearby constellations.
You should try these out next time you see The Big Dipper in the night sky. There are of course other ways that you might find these constellations and as you become more familiar with the night sky you will discover your own tricks and shortcuts to help you hop between constellations.
I would love to hear how you get on with finding these constellations using The Big Dipper. Please feel free to leave your comments below.
All star charts in this post were generated by Starry Night Enthusiast 8 © Copyright Simulation Curriculum Corp. All rights reserved. www.starrynight.com
2 Comments
Rebecca · January 12, 2022 at 6:28 pm
Thanks for this. I could always find Polaris when I was growing up; it was at the top of my Dad’s radio antenna. Now I’m old, live farther north, and have had trouble locating my old familiar stars. I can find Cassiopeia easily, so your article will be a great help.
Mike_Soulby · January 13, 2022 at 5:24 pm
Thanks Rebecca, I am happy you found it helpful and good luck locating them all.