Why do the positions of stars on the celestial sphere need to be updated periodically?

Answer 1

Hi Nazdaran!

Yes they do, due to precession of the equinoxes.

Precession shows itself in three ways:

1) The north and south poles of the sky move against the background of stars. Right now, the north pole points very close to Polaris, while there is no bright star near the south pole. In about 13,000 years, the north pole will point not far from the brilliant star Vega, while the "south star" will be the still-more-brilliant Canopus.

2) The point in the sky where the sun stands on the first day of spring moves. Currently, it is in the constellation Pisces, but in two thousand years the event will happen when the sun is in Aquarius. (Hence, the "Age of Aquarius.")

3) The celestial lines of latitude and longitude will change. In 7,000 years the Southern Cross, which currently cannot be seen north of Key West, Florida will once again be visible from New York, and in another 4,000 after that will actually appear in the skies of southern Greenland. Observers in the north temperate zone will then no longer be able to see at all the brightest star, Sirius (or Canopus), and even the star Capella will then have moved into the Southern Hemisphere.

Precession happens fast enough that the lines of latitude and longitude change just over a normal human lifetime. Over a thousand years, Polaris has moved from being just another northern star to become THE north star. (Think about it! The Vikings did not have Polaris as their north star.)

Answer 2

The stars move around the galaxy and universe centers, just as the earth moves about the center of the solar system (the sun). The all move in different directions so star catalogs must be updated every few years to account for this. While the average person probably won't notice this motion, sensitive sensors that are used in astronomy and on satellites do. It is also true that the nutation or wobbling of the earths axis does contribute to the apparent motion of the stars, but this is more of a shifting of the whole star catalog rather than determining a new position all together.

Answer 3

Predictable movement due to precession, aberration, etc. can be corrected by calculation and doesn't require re-measurement. Stars do move due to their own proper motion, and periodic position measurements track them. Another motivation for re-surveying star positions is to take advantage of improved accuracy. The Hipparcos satellite did that for over a million stars in the 1990s.

Answer 4

1: The stars are in moving through the galaxy like planets in a solar system.

2: The earth's axis is wobbling due to precession.

This is why the dates for the zodiac are about a month out of whack since they were determined 2000 years ago.

Answer 5

This seems to be a problem about gravitiational lensing. The rays from the star are bent toward the sun, slightly. This makes the star seem further from the center of the sun than it otherwise would (i.e. if the sun's mass were smaller).

Answer 6

Precession of the equinoxes, and the proper motion of the stars.

Both were originally discovered by comparing current star charts to ones prepared a couple of thousand years earlier.

Answer 7

Stars move relative to each other. This is very slow but measurable. They can chart back to what the constellations would have looked like in the Roman times and they were quite different

Answer 8

As the Earth spins it wobbles on it's axis - this is called precession.

What this means is that the positions of the stars in the sky relative to our view point appear to move over a period of time.

Answer 9

And nobody has mentioned that the Galaxies are also moving with respect to each other. The Magellenic Clouds have shifted a bit since Magellan first charted them.

Doug

Answer 10

It is because of the stellar parallax as well as due to relative velocity between light coming from that planet & earth.Also because of Earth motion around the sun we observe direct & retrogate motion of the stars.