An Introduction to Star Maps
Perhaps one of the most underrated pieces of kit for a beginner astronomer is the humble star map, an image of how the night sky looks at any given time or location. Although many are available online such as Stellarium or Sky Map, being able to use a physical version is exceptionally useful, especially for the coldest nights where it is difficult to use a phone or when you don’t want to damage your night vision.
Similar to how we use terrestrial maps to find destinations on Earth, a star chart is a map of the night sky and one’s guide to navigating the stars. It represents the stellar dome overhead with only the brightest stars and objects and the constellations.
Since the sky is above our heads, your star chart is designed to be held overhead when you read it. The point directly overhead is called the Zenith and can be found in the centre of the chart (where the higher altitude stars are), this makes the edge of the star map represent the observer’s horizon and the larger dots show the brightest stars. The Meridian on a star map goes straight down from North to South and when you look for things that are culminating (reaching the highest or lowest point from the horizon), look for things on that line. In the Northern Hemisphere, the culmination happens when the star is due South. In addition to dots, there are sometimes grey circles that refer to star clusters and ovals to galaxies. The Milky Way is often shown too, either as an outline or a slightly greyed-out area. Some star maps like the one above also draw in the constellations to make them easier to find when observing.
So how do you find things on a star map? To use the map, make sure you are facing the right direction. Turn the map in your hands such that ‘N’ on the map is oriented towards the north. First, you should look for shapes you recognise and then use them as a guiding point to locate other objects -for example, you can use the Big Dipper which ‘points’ to Polaris or Orion’s Belt, which can be used to locate the constellation Taurus or The Great Square of Pegasus to locate to the connected constellation of Andromeda. Once you locate a constellation in the night sky, try to match it with your star chart. When you’ve found it, orientate the chart so that it matches your view of the night sky.
By using the cardinal points, North, West, East and South, the map indicates the direction in which you should point. Some particularly observant readers may notice that East and West on a sky map are switched around to how you would ordinarily see them on a regular map. To understand why, it helps to imagine lying flat somewhere (eg. a field at the centre of the UK) looking up at the sky with your head facing North and your feet facing South. Now you point with your right hand towards Wales and Ireland, in the West. You can now repeat this with your left arm - now pointing in the direction of East Anglia. Therefore, everything on your right hand is in the West and everything on your left hand is on the East. You could also imagine it from the perspective of an observer at the exact same point but far above the Earth. When at looking down on Earth they see the Americas on the West (with their left hand) and Asia to their East (with their right hand). However, if they were to roll 180 degrees, so they are now lying with their back to Earth, the Americas would be to the right of them and Asia to the left. So, everything they see above them would also reflect that.
On some star maps, there may be a variety of lines drawn across it, the 3 most important being:
The celestial equatior: the “celestial equivalent” of the Earth’s equator -which is ‘projected' from the center of the Earth to become the celestial equator and the Earth's axis is extended to become the celestial axis which its the celestial sphere at the north celestial pole (NCP) and the south celestial pole (SCP). Basically, imagine shrinking the celestial sphere down to the size of the Earth, drawing the equator and North / South poles on it, and then expanding it back out. The resulting markings on the celestial sphere give us the celestial equator, north celestial pole and south celestial pole. Latitude and longitude are projected outward to make the equatorial coordinates, except for some changes. From the equator, the celestial equator will be straight on a star map, since we would be looking at it side-on, but from an observer in Edinburgh at the same time as our equatorial observer above, it would appear curved. As you go north/south away from the equator, the celestial equator will appear increasingly more “rounded”. Finally, if we have a third observer standing on the north pole, the celestial equator becomes a circle which “matches up” with the horizon.
(Reminder on Latitude and Longitude: Latitude is how far above/below the equator a location is, with a 90◦ latitude at the north pole and a 0◦ latitude at the equator. Southern hemisphere locations are denoted either by a negative latitude (e.g. −51◦ ) or by a “S” which means “South” (e.g. 51◦ S). – Longitude is how far east / west the location is from the Greenwich (or Prime) meridian, an imaginary line that goes from the North pole to the South pole, through Greenwich. Longitudes west of the Greenwich meridian are considered negative, and eastern longitudes are considered positive by convention; alternatively, similarly to latitude, you can use “E” or “W” to indicate which side you’re on.)
The ecliptic: a line along which the Sun and (approximately) all the planets of the solar system appear to “move along” over the course of a year (the plane of the Earth's orbit -which is also the Sun's orbit around the Earth -is the ecliptic plane, which cuts the celestial sphere at the ecliptic, meaning the ecliptic pole is perpendicular to the ecliptic plane. The Earth's axis is tilted from the ecliptic pole by about 23.4 degrees in respect to the ecliptic pole. Since the Earth's axis is titled from the ecliptic pole by 23.4 degrees, the Earth's equator and the celestial equator are tilted from the ecliptic by 23.4 degrees. Half of the ecliptic is below the celestial equator, and half of it is above. The ecliptic links very strongly to the zodiac constellations. In fact, the zodiac constellations are defined as the constellations that are on the ecliptic line. As the Sun moves all the way along the ecliptic each year, it slowly moves through all 12 zodiac constellations, spending approximately one month in each one. The month corresponding to each zodiac constellation is the month when the Sun is blocking that constellation from view. Ironically, this means that the zodiacal constellation for each month is not actually visible during that month! Rather, the zodiacal constellations on the opposite side are most clearly visible. For example, take August, when Leo is not visible. Imagine that it is midnight and the observer is facing directly away from the Sun. It is Aquarius that will be directly visible to the observer at that time, because Aquarius is directly opposite Leo in the sky
The galactic equator: Our Solar System is within the Milky Way galaxy, which is a spiral galaxy and therefore has a relatively flat shape. This means that when you look into the sky, there are some directions pointing “out” of the plane of the galaxy, and some directions pointing “through” the galactic disk. The galactic disk is inclined at an angle to both the celestial equator and the ecliptic, and so it is the “galactic equator,” which shows where the plane of the Milky Way galaxy is in the sky, as viewed from Earth.
To better understand this, it is best to use the concept of the “celestial sphere”, which is a huge, imaginary sphere surrounding the Earth as a way of imagining all the stars, objects, constellations etc. as features “painted” on the inside of this sphere. It is important to note that this does “approximate” them as all being the same distance away but this works fine because when you’re figuring out where to point to something in the night sky, it doesn’t matter how far away it is, only what direction it is in! So what does this have to do with lines? Lines are also something that can be drawn on the inside of a huge sphere. And when you do this, the lines are actually circles (and the most important ones have diameters equal to the diameter of our celestial sphere - but don’t worry about any of this too much, it is all just a bit of background explanation!). Depending on where you are on Earth, these lines can appear very curved or straight on a star map.
Some things to look out for using a star map:
The Winter Hexagon:
Constellations: Orion, Taurus, Auriga, Gemini, Canis Minor, Canis Major
Stars: Betelgeuse, Rigel, Aldebaran, Capella, Castor, Pollux, Procyon, Sirius
Asterisms (An asterism is a group of stars that form a recognizable shape that is not
officially a constellation. An asterism may be part of a constellation (e.g. the Big Dipper) or
can consist of stars from several constellations (e.g. the Winter Hexagon). Another asterism
to notice is the Winter Triangle, formed by Betelgeuse, Sirius, and Procyon.) : Winter Hexagon, Winter Triangle
Objects (A deep-sky object is the umbrella term given for any nebula, galaxy, star cluster etc. that you might use a telescope to observe): M1 Crab Nebula, M45 Pleiades, M42 Orion Nebula
Key features:
– Orion’s belt: the three stars in Orion that are very close together. Once you’ve found
Orion, you can look to his left shoulder for the star Betelgeuse and to his right knee
for the star Rigel.
– Canis Major (“Big Dog”) and Canis Minor (“Little Dog”) are Orion’s hunting dogs, so
look behind Orion for them. Sirius is the brightest star in the night sky, so if in doubt,
look for the brightest star in the region to find Sirius.
– Gemini represents twins, and you may know that this is a zodiac constellation (why
zodiac constellations are important will be elaborated on later). The two stars Castor
and Pollux can be easy to mix up, but there is a memory trick to avoid this: Castor
is closer to the star Capella, whilst Pollux is closer to the star Procyon.
13– Auriga represents a charioteer but really just looks like a hexagon. The brightest star
(biggest dot) in this hexagon is the star Capella.
– Taurus is another zodiac constellation. Look for the distinctive small “triangle” of stars
that represent its head.
The Winter Hexagon and Triangle
2. The Summer Triangle:
Constellations: Cygnus, Lyra, Aquila
Stars: Deneb, Vega, Altair
Asterisms: Summer Triangle
Objects: M57 Ring Nebula
In same way that the Winter Hexagon and Winter Triangle asterisms are given those names because they are visible from the northern hemisphere at night in the winter, the Summer Triangle is especially visible in the summer.
3. Some intresting Zodiac Constellations:
Cancer, which is between Gemini, and Libra both are fairly dim and can be tricky to find. However, by knowing which constellations they are between, you can know their approximate location on a star map.
Libra, which is between Virgo and Scorpius is brighter than Cancer but can still be tricky to spot, so by using the positions of Virgo and/or Scorpius to find Libra. You can use the Big Dipper (if visible) to find Virgo using the saying “arc to Arcturus, speed on the Spica” (Spica is the brightest star in Virgo), and you can also use the constellation Leo, which has a fairly recognisable “coat-hanger” shape.
Sagittarius, whilst supposed to be an archer but looks a lot more like a teapot (hence contains the famous “Teapot” asterism) and Scorpius - the scorpion are both very bright and distinctive constellations in the sky, making them great reference points for finding other things.
For Capricornus and Aquarius, by knowingwhere they are relative to Pegasus, Aquila, and/or any of the other constellations that you already know can help you find them in the night sky and on a star map. For example,
– Whilst Pisces is directly below the body of the Pegasus, Aquarius is below the head.
– The left wing of Aquila points towards Capricornus.
4. Other well known asterisms and constellations:
Constellations: Perseus, Andromeda, Pegasus, Cassiopeia, Aries, Pisces
Stars: Alpheratz
Asterisms: Great Square of Pegasus
Objects: M31 Andromeda Galaxy, M76 Little Dumbbell Nebula
Despite the name, within the Great Square of Pegasus, only three stars from this square are from Pegasus, with the top left corner star being Alpheratz, a star in Andromeda. Pisces wraps around the bottom left corner of the Great Square.
Constellations: Ursa Major, Ursa Minor, Bootes, Virgo, Leo
Stars: Polaris, Arcturus, Spica
Asterisms: The Big Dipper
Objects: N/A
The Big Dipper is arguably the most well-known asterism in the Northern Hemisphere. All of the stars in the Big Dipper are from the constellation Ursa Major (“Big Bear”). This asterism is incredibly useful for finding the stars Polaris, Arcturus and Spica.
Once you have noticed the distinctive “saucepan” shape of the Big Dipper, you can use the saying “arc to Arcturus” to find the star Arcturus as shown. If you carry on further, the only particularly bright star in that region is Spica, hence the next saying: “speed on to Spica”. Finally, if you use the opposite end of the Big Dipper, you can also use the two stars on the front of the “pot” section as pointers to find Polaris.
Many thanks for reading and I hope that you will join us next month for another info-packed explainer. If you have any questions about this or any other topic or want to share your astronomy stories or astrophotography make sure to put it down in the comments or @ @EveCarruthers_ or @WeeklySpaceman on X and you will be featured next month!
