The Science Behind Night Sky Star Maps

How To Verify Your Star Map Is Accurate.

From time to time a customer creates a star map and for one reason or another, doubts the accuracy of the print. 

We first need to be clear that our star maps have never been and will never be a "generic" print: We individually and precisely render each star map for the chosen time and place and guarantee accuracy! We are very proud of our star maps and so we hate to see people concerned that their print might be incorrect, so we will always do our best to reassure you in any way we can.

How to verify your star map is accurate:

The first thing you can do is to compare with a third party, to see that our maps are accurate. SkyAndTelescope, a popular astronomy magazine (with no affiliation to TheNightSky) has an interactive sky chart that you can use to verify your map details:

You can also download free astronomy software such as Stellarium ( to explore the night sky in a more interactive way and to compare against your print.

It’s one thing to show that third parties agree that the print is accurate, but that doesn’t answer the question that may have raised concern in the first place: why the prints appear as they do and how could a pair of maps which appear very similar, despite coming from different times or from different locations? Or conversely, how could two star maps look different, despite coming from apparently the same date and location.

These cases are rare but perfectly possible. We will discuss these and other specific cases below, but first we need to describe (the somewhat complicated mechanics of) how the stars move and the factors that influence which stars you see in the Night Sky. 

How the stars move

We say that the stars move, but in reality, the stars don't move relative to one another by any measure that we can see the change visually here on Earth. What's actually happening is that we - here on spaceship earth - are moving beneath them. To try to visualize this you can try to imagine the sun hanging in space and the earth slowing orbiting around it. Surrounding that, you should then imagine a huge globe with the stars painted upon it - a fixed background with the sun in the middle and the earth slowly rolling around it.

We call this imaginary globe of stars the "Celestial Sphere". Hopefully, by now you're now imagining something like this image (via Google Images): 

Now try to visualize you are a tiny figure standing on the earth. You can imagine that if you were to walk to the top of our little green Earth and look straight up, you would be looking out at a different view versus if you walked to the bottom (where looking straight up would be looking down at the stars at the bottom of our imaginary image of the Celestial Sphere). In this way, we see that location affects the stars we see above us each night.

Of course, we're not running laps around the earth each day! More usually we stay - on a global scale - in more or less the same location, but the earth is spinning beneath us and carrying us along with it.

The earth rotates every 24 hours so that at noon you're looking straight up at the sun, toward the center of our imaginary celestial sphere. At midnight, looking straight up means looking directly away from the sun and out into the heavens. As the earth rotates, this "straight up" position away points out at a slightly different set of stars, moment to moment, hour to hour, so that they appear to move across our sky during the night. At night we see one half of the celestial sphere and as the earth turns us to daytime, the brightness of the sun (and in part the sun itself) blocks our view of the stars on the other side of the Celestial Sphere, beyond the sun in the center of our model. 

As days and months pass, the earth also moves around the sun so that each night at midnight we look straight up and away from the sun and out at a slightly different piece of sky. In 6 months we move to the opposite side of the sun, looking out at the opposite side of the celestial sphere and after 12 months we have completed an orbit, back to the original position.

So you see that the stars change on an annual cycle. This is one of the nice things about the star maps. The stars we see on a date this year are the same as those we see 1 year later, 10 years later or 100 years later (ignoring for the simplicity of this explaination, some slower and more subtle effects that occur over centuries).

So we can now see how time affects the positions of the stars through the earth’s rotation. The second factor that influences the stars we see, is our location on Earth.

The star map displays the entire dome of the Night Sky as you see it above you. The edge of the cirlce represents the horizon and with the center of the circle is the point straight up above you. The stars to your North are in the top half of the map, the South to the bottom, East to the left (you're looking up) and West to the right. The circle displays half the celestial sphere - on any cloudless night with a clear horizon you can look out into half the universe! That’s 180 degrees of sky in all directions.

If you think about the earth as a little globe floating inside the celestial sphere, you know that at any time half of the celestial sphere is above you - the rest is behind the globe you're standing on. If you change location and move to the other side of the globe, you would see the other half. Moving around the globe would move the stars by an equivalent amount - remember the stars aren’t moving - you are!

How can two star maps from different locations look the same?

Sometimes we are asked why a star map from one location looks like another. Because we're looking at half of the celestial sphere, the scale of the map is very large - horizon to horizon, the amount of sky you can see above you is massive, much like if you were to look at half of the globe. Often the issue is simply that the difference is too subtle at the scale of the map. For example, Washington and New York are separated by only 7 degrees of latitude: a minor shift within the 180 degree expanse of the star map! 

A more interesting example of this can be found in a shift from New York to San Francisco or Sydney to Perth. In the latter case, the North-South separation is less than 2 degrees, however the cities are separated East/West by almost 4,000 kilometers on opposite ends of the Australian continent! So how for the same date, might their star maps show the same skies?

The answer in this instance, comes with time. By default, we render star maps at 10 PM in the evening, since these are the stars and constellations that people remember seeing and ask us about on the date they wish to remember. And strictly speaking, rendering at midnight would show the stars for the following day!

The time chosen is local time, so if you were to render two star maps for the same date for example in New York and San Francisco, the stars rendered in the New York map would be at 10 PM New York time, and the stars in the San Francisco maps would be for 10 PM San Francisco time, 3 hours later! During this time, the earth has rolled around underneath the stars, so that San Francisco is looking up at the same stars that were above New York earlier. 

You can render the stars for the same moment in time by making appropriate adjustments to the time - in this example, setting the New York time to 11 PM and the San Francisco time to 8 PM, for example, which would give you quite different maps (rotated 48 degrees, their difference in longitude). You can select a specific time on the desktop computer/laptop version of our site, by expanding the "Advanced Options" section.

“How could two star maps look different for the same date” (or “My star map doesn't match the star map I ordered 1 year ago”)

It's a great compliment that many of our customers return to buy second and third copies of the prints they have previously purchased. Sometimes, however, they note with alarm that the stars have changed between their earlier print. This is nothing to be alarmed about! 

We have moved our default time over the months before settling on 10 PM. Typically, the earlier print is a slightly earlier time and we have found the later time of 10 PM to be a better default because for some Northern and Southern extremes, the sunset occurs very late in the evening during the summer months.

We would like to encourage our customers to pay attention to the time if they are ordering repeat prints and to read these notes carefully to understand that this is a change in time and does not mean that the map is inaccurate. However, if your print arrives with a time you were not expecting then as with any customer concern, we will do whatever we can to to make it right and make sure you are completely satisfied with your print.

How could two star maps months apart look similar?

Since providing the option to select a specific time to render the stars, we've had some interesting cases where customers have expressed concern about star maps motnths apart looking very similar. In one fascinating case, a mother's two children were born two and half months apart under a virtually identical sky! In all cases that we have addressed thus far, each has been the result of the combination of the date, in the orbit on the earth around the sun, and the time, in the earth's rotation.

If you remember again our imaginary image of the earth sitting inside a huge globe of stars: We know that the earth rotates around the sun every 12 months, so in 6 months at midnight we are looking out at a half of the celestial sphere immediately opposite the site we look out at 6 months later. We also know that the earth rotates every 24 hours, so after 12 hours we're pointing back towards the side we pointed at 6 months ago! So for example we point at the same stars at midday in January would at midnight in July!

Obviously, in this case the sun should be blocking one or the other views and we wouldn't necessarily encourage you generate star maps for the middle of the day (although you might remember that the stars are still there even when you can't see them)! Usually in these cases, the difference is a matter of a few months or between dawn and dusk. At midnight one month to the next, our view of the celestial sphere will have moved 30 degrees. Since each hour the earth rotates 15 degrees, 1 month is equivalent to 2 hours of rotation - put another way, an 6 AM map one month will look virtually identical to a map drawn at 4 AM map a month later.

Recommended Reading

If you'd like to read some more about the mechanics of how the stars move and learn about how to locate constellations in the Night Sky, then I'd recommend the book called "The Stars", by H. A. Rey.

If you have any concerns about the accuracy of our maps that we do not address here, or wish some point to be clarified, then please do let us know. We'll always do our best to address your concerns and to make sure you are as proud and happy as we are in the quality and accuracy of our star maps. We can also send you a gif file if you want to show you the difference.