The APL Astronomy Club
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Learning about Astronomy
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Horizon -
A point seen from the observer, perpendicular to an imaginary point straight up (the Zenith), that limits the distance the observer can see while on the Earth or solid planetary body.
Zenith -
The point straight up from the observer and equal distance from the Horizon. Elevation is measured above the horizon to Zenith.
Equator -
The longest straight line measured around the Earth and equally placed between the Noth and South poles. On Earth it's measured in degrees (i.e., +45 Deg. or 45 Deg. North), in the night sky and on Star Charts it's measured in Hour, Minutes, Seconds & fractional seconds (i.e., 11h 32m 21.5s).
Celestial Equator -
An imaginary point in the sky directly above the Earth's Equator.
Right Ascension (RA) -
This is the Celestial equivalent of Longitude, measured in Hours, Minutes and Seconds starting at zero (0) in Greenwich, England increasing easterly up to 24 hours (actually 23h 59m 59.99s).
Declination (Dec) -
Starting at the Celestial Equator it's measured in degrees from 0 degrees to +90 degrees or -90 degrees.
Ecliptic Plane -
The ecliptic plane is defined as the imaginary plane containing the Earth's orbit around the Sun. The imaginary great circle in the sky where all the visible Planets, the Moon and the Sun pass during their apparent motion nightly.
Azimuth -
The angle from the observes point of view, measured clockwise in degrees from North around the Horizon to a point directly below the celestial object.
Altitude (Elevation) -
The angle of an object measured in degrees from the horizon toward the Zenith.
Celestial Meridian -
The imagary line or great circle that passing from North through the Zenith to the South. This line divides the sky into two halves, the Eastern and Western. When a star, planet, our Moon or Sun crosses the Meridian it's said to have "Culminate(s,d)".
Circumpolar -
Stars that circle the North Polar and are always in sight from the observes point-of-view.
North Polar (Star) -
The visible star nearest to a point in the night sky that appears not to move through the course of the night. The nearest star in the Northern Hemisphere is Polaris, nearest in the Southern Hemisphere is is not visible by the naked eye.
Northern / Southern Skies -
The stars that are visible in the night sky given the Latitude of the Observers Point-of-View.
Summer / Winter Equinox -
Seasons are due to the Earth's 23 1/2 degrees tilt on its axis, its rotation and revolution around the Sun in an elliptical orbit. At Equinox the Sun rises directly East and sets directly West. This progression can be seen in an Analemma, the unusual "figure 8" shape found in various forms on Sun Dials.
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This web site was updated on 08/10/99
For additional information contact
Bernie Kluga President, Mark Simpson VP , Carol Bates, Treasurer, Nick Beser, Secretary
APL Amateur Astronomy Club, 443-778-7257 or 240-228-7257
Copyright 1996, 1997, 1998,1999 by JHU/APL Amateur Astronomy Club. All rights reserved.
