You have noticed that even on this website, there are posts dedicated to showcasing the constellations best seen during each season. Beginner level observers will notice that there are stars that are prominent during some months, but at other months they are barely visible at all. Have you also noticed that the stars you see after sunset are not the same stars you see before sunrise? This all has to do with Earth’s true rotation compared to its orbit around our Sun.
Earth’s Rotation Relative to the Stars is Four Minutes Faster than Relative to the Sun!
Earth’s true rotation is in fact every 23 hours, 56 minutes, and 4 seconds, not 24 hours. This is the rotation relative to the stars, not to the Sun! We call this the Sidereal Rotation. As a comparison, Earth’s Solar Day – the rotation relative to the Sun, is 24 hours. That’s almost a four minute difference! It may not seem like much, but it is this difference that keeps our solar days intact, and ensures that every day around noon, the Sun is crossing the meridian.
In this picture, the location where the Sun is crossing the meridian (local noon) is represented by the line from Earth to Sun, while the point where Earth does a complete rotation is represented by Point A. As Earth moves up in its orbit and does a full rotation, it completes its rotation in 23 hr 56 mins, yet as you can see, Point A isn’t pointed at the Sun yet. It’s still about 4 minutes before local noon. Only after those four minutes pass does local noon occur, and at the same time, Earth is already four minutes into its next rotation!
Rinse, lather, repeat!
But Then Why do the Stars Change After a Few Months?
We’re getting there!
You see, as the sidereal rotation is relative to the stars, that means that every time Earth completes a rotation, it’s directly lined up with a fixed star in the sky. You can observe this with any star in the sky!
The easiest way to do this is to pick a bright star at night, and use the meridian line as a point of reference for when Earth has completed its rotation – though you can use any point you wish.
Let’s say you pick a star that is crossing the meridian line at 9:00. Go outside the next day, and observe the same star crossing the meridian line; what do you notice? The same star has crossed the meridian at 8:56, four minutes faster. Go outside the next day, and you will see the same star has crossed the meridian at 8:52.
Now what happens if you focus on the same star’s position at 9:00? you notice that with each day at 9:00, the star shifts west a tad, and depending on how north or south the position of the star is, you definitely notice that after a few months go by, it’s in a completely different spot in the sky, or it has disappeared below the horizon. By now, you also notice the stars above are different than the ones you remember from a few months ago!
Because the Earth’s sidereal rotation is four minutes faster than a standard solar clock, that means the stars gradually shift every day, and after a few months, new stars appear to move in while others begin to set below the horizon.
So Then Why Do We Have Winter Stars and Summer Stars?
As Earth completes its orbit, the night side is facing different constellations. Constellations are given their seasonal category based on their visibility in the evening hours after sunset.
The Sun’s position among the fixed stars appears to shift day by day due to Earth’s orbit. Take December for example, as the Sun is in front of the constellations that we see in June.
The constellation Orion is visible all night in December, as it is directly opposite the Sun compared to Earth. But in June, the Sun will be in front of that sector of the sky, making it impossible for you to see Orion at night because it’s lost in the Sun’s glare.
But Wait… I saw a Winter Constellation in the sky during August!
On any given night due to the Earth’s rotation, you can see up to HALF of the stars, six month’s worth, visible from earth during the night – the other half is lost in the Sun’s glare during the day. After sunset, while you do see your seasonal stars in the sky during the evening hours, you also see the stars from the previous season setting in the west. As the night progresses, the stars you see in the evening begin to set after midnight, and the next season’s stars are visible in the sky. By the time the night is over and the Sun is about to rise, you start seeing stars that Earth’s night side will be directly facing in 6 months!
For example, in April, you can see Orion setting in the west after sunset, signifying that Winter is over and Spring is here! In May, Orion starts getting lost in the Sun’s glare, and during the months of May, June, and most of July, Orion cannot be seen at night due to its position near the Sun. Starting late July into August, the Sun has moved far enough away from Orion that you can start seeing it rise low in the east during the early morning hours just before sunrise.
So don’t be surprised to see the autumn and winter constellations in the sky while you’re out watching the Perseids Meteor shower in August!