Lewis Bermap age 11, of Huntsville, Alabama, for his question:
What is an equatorial telescope mounting?
A giant telescope weighs many tons and naturally it must be mounted on sturdy. supports. However, the mounting must be movable so that an observer can aim the instrument at different parts of the sky. This tricky job is done by a complex mechanism called an equatorial mounting.
Chances are, the first telescope you own is a simple spy glass in a longish tube. Maybe it came with a tripod stand to keep it steady and leave your hands free to aim and focus your instrument. You must keep changing the aim because the object you are watching continuously drifts across your field of vision and out of range. After a spell of this sky spying, you must rest your weary muscles and work a crick or two out of your neck. This pause gives you time to wonder how astronomers support their massive telescopes, aim them and kip them trained on moving celestial targets.
A giant telescope must be able to point at any spot in the heavens. To do so, its massive tube must be able to move in two general directions. There is a wheel to make its roving eye inch over the sky from pole to pole. There is another wheel that moves it along a great arc parallel with the equator from the eastern to the western horizon. This mechanism is called an equatorial mounting because it uses the equatorial system of coordinates.
Astronomers chart the sky on the inside of an imaginary dome called the celestial sphere? It has a celestial equator, celestial poles and lines of latitude and longitude directly above these co ordinates on our globe. An astronomer knows the exact co ordinates of the heavenly body he wishes to observe or photograph. By turning the two wheels on the equatorial mounting, he moves the giant telescope until it aims right at the target.
This special mounting has two sturdy rods fixed together at right angles. Each is called an axis because it can be rotated. One is the polar axis, perfectly aligned with the earth's axis and pointing to the celestial pole. This axis can be made to rotate along any arc parallel with the equator. Fixed to the polar axis at right angles is the declination axis and this rod is fixed to the massive telescope. The declination axis is parallel with the equator, which means that it rotates between the poles.
A wheel fixed to each axis is marked with degrees. When the wheel is turned, the axis rotates and the giant telescope turns with it. The astronomer turns both wheels to get the right co ordinates. The polar axis points the telescope to the proper degree of celestial latitude. The declination axis points it to the proper degree of celestial longitude. However, as the earth rotates, the target in the telescope moves out of range. So the astronomer starts a clockwork mechanism that moves the equatorial mounting to keep pace with the spinning earth.
The polar axis of an equatorial mounting must parallel the earth's axis. But the earth's surface is curved and this parallel varies from latitude to latitude. The mounting must be adjusted to the location of the telescope. If the telescope stands on latitude 40 degrees North, its polar axis is inclined at 40 degrees to the horizon. A big telescope in Huntsville, Alabama, would have its polar axis inclined at almost 35 degrees from the horizon. This is about the same height as Polaris above the Huntsville horizon.