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.
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.
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.