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A glossary of the terms in the Observatory


Altitude:
Angle above the horizon, in degrees. 0° is the horizon, 90° the zenith.
Azimuth:
Angle from the North, from 0° to 360°, measured towards the East.
So, 90° is East, 180° South, 270° West, etc.
Celestial Poles:
Intersections between the Earth's axis of rotation and the Celestial Sphere.
Celestial Equator:
Projection of the Earth's Equator onto the Celestial Sphere.
Celestial Sphere:
An imaginary sphere surrounding the Earth, onto which we consider all other celestial objects to be pasted. Stars change their positions on this sphere very slowly. For most purposes, in the time scale of decades, they can be considered fixed. The movement of the stars that we see nightly can then be thought of as a uniform rotation of the Celestial Sphere.
CDT:
US Central Standard Time, i.e. UT+6.
Dec:
See Declination.
Declination:
The equivalent of Latitude, on the Celestial Sphere.
Measured in degrees, from 0° at the celestial equator to 90° at the celestial North Pole and 90°S at the South one. The North Star is at almost 90°N.
Equator:
Great circle on a sphere, at the same distance between the two poles.
Equatorial Grid:
A grid drawn on the (imaginary) sphere of the stars (the celestial sphere).
We use one with lines every 10° (from the Poles) and 1 hour (in the other direction).
Field of View:
Angle that the image covers, from side to side.
Normal humans pay attention to a Field of View of about 45° (although our eyes actually cover a much larger angle).
FoV:
See Field of View.
Latitude:
Angle between the Equator and a point on the surface of the Earth.
Usually given as an angle between 0° and 90°, and a letter (N or S) indicating whether it is to the North, or South of the Equator.
Longitude:
Angle between the reference meridian (at Greenwich, England) and a point on the surface of the Earth.
Usually given as an angle between 0° and 180°, and a letter (E or W) indicating whether it is to the East, or West, of the Greenwich Meridian.
Poles:
Points at the intersection between the axis of rotation of a sphere and its surface. There are two of them, usually called North and South Poles.
R.A.:
See Right Ascension.
Right Ascension:
The equivalent of Longitude on the Celestial Sphere.
In technical terms, it is the angle between the Vernal Equinox and the celestial meridian of the object, measured to the east.
Usually measured in hours, minutes and seconds, though sometimes degrees are used, too.
Time: Who knows what time really is?
Whatever time is, in the Orrery Sky pages we refer to local time (i.e. of the location from which one wants to observe the sky).
In the other pages, we use US Central Standard Time (which is Universal Time (or Greenwich Mean Time) +6 hours).
UT:
Universal Time, i.e., the time at Greenwich, England.
Vernal Equinox: The point on the Celestial Sphere where the Ecliptic crosses the Celestial Equator towards the North. It is used as the origin of Right Ascension measurements.
Zenith:
Point on the celestial sphere directly above the observer's head.
Zoom: A factor of magnification of the image.
Zoom 1 corresponds roughly to what we normally see by the naked eye (FoV of 45°). Larger values cover smaller angles, and objects appear larger. Use values between 0 and 1 (0.5, for example) to get images of wider areas.


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