Star Set...?

Question

Using a good digital watch, get the exact time when a bright star goes behind a distant building or tower. A day later time the disappearance again. Sighting over a nail fixed in a window sash will help you return your eye to the same location for each sighting. It will be found that:

a) the star disappears at the same time each night
b) the star disappears a little earlier each night
c) the star disappears a little later each night.

'Twas noontide of summer,
And mid-time of night;
And stars, in their orbits,
Shone pale, thro' the light
Of the brighter, cold moon,
'Mid planets her slaves,
Herself in the Heavens,
Her beam on the waves.
I gazed awhile
On her cold smile;
Too cold- too cold for me-
There pass'd, as a shroud,
A fleecy cloud,
And I turned away to thee,
Proud Evening Star,
In thy glory afar,
And dearer thy beam shall be;
For joy to my heart
Is the proud part
Thou bearest in Heaven at night,
And more I admire
Thy distant fire,
Than that colder, lo lowly light.

Edgar Allan Poe, 1827

If true...why?

The answer is: b. The motion of the stars in the night sky is due to the earth spinning, so the stars, like the sun, should seem to go around the earth every twenty-four hours, but not quite. Why not quite? Because you have two circular motions: one around the earth every day and one around the sun every year.

If the earth kept one side facing the sun all the time the star would go around the earth once every year. But the earth does not keep one side facing the sun. It spins around so one side does not always face the sun. It spins so fast that you see the sun go around about 365 times each year. So you also see the star go around 365 times each year. NO. You see the star go around 366 times! Why the extra time? Because it would go around once each year even if the earth kept one side always facing the sun. Remember, there are two circular motions, and the total circular motion is the sum of both.

How do you know the circular motions should be added together? Maybe one should be subtracted from the other. The two motions add because thee earth’s daily spin and yearly orbit both circle the same way. The sun also spins the same way. The earth’s spin and orbital motion probably came with the material taken from the sun when the earth was created. That’s why all circle the same way.

So the star must go around the sky a little faster than the sun. The sun goes around once in about 24 hours so the star goes around once in a little less than 24 hours. How much less? The sun goes around the sky in about 1,440 minutes (1,440 minutes=24 hours x 60 minutes per hour). The star must go around the sky a bit faster so that after one year it has made it around one more time than the sun. In other words the star has to get in an extra turn in one year. If a normal turn (sun turn) takes 1,440 minutes and if you cut the time by 4 minutes (4 times 365 approximately equals 1,440)...

then after 365 turns (one year) you will be one turn ahead.

So stars don’t set (or rise) at the same time each night. Each night they come up (and go down) four minutes earlier than they did the night before. That adds up to roughly half an hour in a week. So at the same time of night you don’t always see the same stars in the sky, even though at the same time of day (say noon) you always see the same star – the sun. It is this effect that causes the constellations in the winter night to be different from those in the summer night.

In 1931 a radio engineer named Jansky who worked for the Bell Telephone Labs picked up radio noise on a particularly sensitive short wave receiver at about the same time each day. No one could tell or even guess where the radio noise came from. Then Jansky noted that the noise came four minutes earlier each day. He said that means the radio noise must be coming from the stars.

For years no one would believe it, but he had in fact discovered the first extraterrestrial radio source – the center of the Milky Way.

Answer 1

The sidereal day is 4 minutes shorter than the solar day.

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Green to green, red to red,
all is well, go ahead.

When in trouble or in doubt,
turn in circles, scream and shout.