| MadSci Network: Astronomy |
We generally don't use centimeters to measure distances in the night sky. The measurement would vary depending on how far away from your eyes you held the ruler. Since each person would hold the ruler at a different distance from their eyes, each would get different measurements. When I hold a ruler at arm's length, I measure the distance as about 22.5 centimeters. My son, who has longer arms, measured the distance as 23 centimeters. I realize that this not a big difference, but it would be greater if a child were to try it. Instead of centimeters, we use degrees to measure the distances from one object to another. A person's hand has a width of about 10 degrees when held at arms length. Sirius is about 22 degrees from the closest star in Orion's belt (Alnitak).
For the second part of your question, we need to talk about how we find things in the sky. On Earth we use latitude and longitude to locate anything on Earth. We use something very similar in the sky, right ascension and declination. Right ascension is the east-west measurement, and it ranges from 0 to 24 hours. (It is measured in hours because it takes 24 hours for the Earth to rotate once on its axis or the sky to appear to rotate overhead.) Declination is the north-south measurement and goes from 0 degrees to 90 degrees north and south. Right ascension is zero at the point in the sky where the celestial equator and the ecliptic meet. It is now in the constellation Pisces. The ecliptic is the line or path the sun apparently traces across the celestial sphere. (Right ascension 0 hours is the point in the sky where the sun is on the Vernal Equinox in March each year) The celestial equator is directly above the earth’s equator. The coordinates for Sirius are about:
The negative sign means south of the celestial equator. The nearest star in Orion’s belt is Alnitak with coordinates of about:
The farthest star in Orion’s Belt is Mintaka, and it lies almost on the celestial equator.
Now to answer the second part to your question. From the North Star Sirius lies 106 degrees to the south and 15 degrees farther south than Alnitak.
To answer the third part of your question we need to talk about the motions of the stars. All stars are moving in the celestial sphere. Some move a lot over the years and some will not apparently move for centuries. Sirius and all the stars in Orion are moving. Sirius has moved about 44 minutes in the last 2000 years. In ten thousand years this would be almost 4 degrees. This is not much. The stars in Orion’s Belt have moved only a fraction of that distance in the last ten thousand years. So the distance between Sirius and Alnitak has changed little in the last ten thousand years.
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