Additional Reading at www.astronomynotes.com:
During the day, we can see the Sun, but the bright daylight sky prevents us from seeing most other objects in the sky (on some days you can see the Moon during the day, and if you know where to look you can also sometimes see Venus).
As a thought experiment, think about what you might see if you were able to see the Sun and the stars in the sky during the daytime simultaneously.
- What stars would you see behind the Sun?
- Would you always see the same stars behind the Sun?
Test this with Starry Night!
- Open up Starry Night, set it for Sunrise, and set the time flow rate to 1 hour.
- Under the View menu or using the options tab, you can select "Hide Daylight," which will allow you to see the stars even when the Sun is up.
- If you want, to help guide your eye, you can also turn on the constellation stick figures using the View menu, the Options tab, or just by typing the letter "k" on the keyboard.
- Now, step through time one hour at a time by hitting the step forward button. Take note of the Sun's path and its position with respect to the stars.
Let's look at two movies made with Starry Night. The first illustrates the path of the Sun during one day (Sunrise to Sunset), following the instructions listed prior.
The second illustrates the position of the Sun at noon eastern on June 21, September 21, December 21, and March 21.
If you could see the Sun and the stars simultaneously, you would see that during the course of one day, the Sun would be inside one constellation (to be more specific, one of the constellations of the Zodiac). To be even more specific, realize that the constellations are made up of stars far in the background, so when we say the Sun is "inside" a constellation, we mean that we are seeing the Sun in projection in front of a specific group of distant stars.
In the first of the two movies, notice the Sun's position relative to the constellation Virgo at 7:00 AM, noon and 6:00 PM. As we discussed at the beginning of the lesson, it is the rotation of the Earth that causes the Sun and the stars to move across the sky, so we should expect that the Sun and the stars should both appear to move at the same rate. Thus, the Sun will be seen inside of the same constellation during the entire day. That is, if the Sun appears to be in the constellation of Gemini at dawn, then it will still be in Gemini at noon and at Sunset.
This is mostly correct, however, there is one effect that we are neglecting to take into consideration. The Earth isn’t just rotating in a fixed spot in space. The Earth is also orbiting around the Sun. In one year, the Earth will make a complete trip around the Sun. So, in December, the Earth will be on one side of the Sun, and six months later, in June, it will be on the opposite side of the Sun.
The second Starry Night movie shown above demonstrates that in December, when the Earth is facing the Sun, the constellation behind the Sun is Sagittarius. Twelve hours later, when the Earth has rotated so that it is night, the Earth is facing directly away from the Sun, towards the constellation of Gemini. In June, the situation is completely reversed because the Earth is on the opposite side of the Sun. The constellation behind the Sun at noon in June is Gemini, and twelve hours later, when the Earth is facing directly away from the Sun, it is pointed towards the constellation of Sagittarius.
This is reasonably easy to visualize when you think of the extreme case of the differences in the position of the Earth six months apart, but what happens on a day to day basis? The way to visualize it is as follows. The stars are so far away from the Earth that, again, for our purposes, we can consider them to be fixed. We know that the rotation of the Earth causes stars to appear to make circles or arcs on the sky that start in the east and move westward. A natural question to ask is, “How long does it take for star A to appear in the same spot in the sky one day later?” That is, let’s say that star A is “transiting your meridian” (this means that if you draw the imaginary line on the sky that connects due North to due South, the star is passing this line at this particular instant in time), how long will it be until star A transits your meridian the next time? You may be tempted to say 24 hours, but the correct answer is 23 hours and 56 minutes! If you do the same exercise for the Sun—that is, if you calculate the time between successive transits of the Sun—it is 24 hours (although it does vary over the course of the year, and some days are slightly longer and others are slightly shorter than 24 hours).
The length of time between transits for a star (any star) is called a Sidereal Day, and the length of time between transits for the Sun is called a Solar Day. The difference is caused by the slow drift of the Earth around the Sun. Because the Earth has moved 1/365th of the way around the Sun in a day, it has to rotate more than 360 degrees in order for the Sun to appear in the same part of the sky (e.g., transiting the meridian) as it did yesterday. However, since the stars are so far away, the Earth’s orbit around the Sun doesn’t affect their apparent position in the sky, so the Earth only needs to rotate 360 degrees in order for them to appear in the same part of the sky. Because of this effect, the Sun appears to slowly drift eastward compared to the background stars, and the cumulative effect of this drift is that the Sun will appear to be in Gemini in June and Sagittarius in December.
Note in the figure above that when the Earth rotates 360 degrees, it goes from position 1 to position 2, and a distant star will appear to be in the same position as seen from Earth. However, the Earth has to go from position 1 to position 3 for the Sun to appear in the same position.
Test this with Starry Night!
- Open up Starry Night, and set it for a time when it is completely dark, say 11:00 PM.
- Using the View menu turn on the Local Meridian line.
- Either adjust the time slightly or let time flow forward until a bright star is on the meridian.
- Now, from the time flow rate box, select "days" as the time step.
- If you click on the forward button, you should see that each day at the same time, your bright star that started on the meridian will get further and further from the meridian.
- Next, click the backward button so that the bright star is back on the meridian.
- Now, change the time step to be "sidereal days."
- Now, if you click the forward button, your star should remain on the meridian without moving each time you click.