Green Globe

COMMON AMATEUR ASTRONOMY TERMS

The user is assisted in exploiting the formulas found in this book through usage notes, definitions, and examples provided throughout the Astro Functions and Methods. This sheet lists some common terms and concepts used throughout the work.

UT is Universal Time, which is the standard time at the prime meridian (0-degrees longitude) running through Greenwich England. UT times are given on a 24-hour clock. In the Americas a number of hours must be added to local time to calculate UT. In the continental USA the standard corrections are +5 (Eastern), +6 (Central), +7 (Mountain), and +8 (Pacific) hours. Add one hour less when daylight savings is in effect. Note that, if this addition causes the time to pass midnight (exceeds 24-hours) you must increment your calendar date. For instance, Central Standard Time (CST) is 6-hours behind UT, if it were 8:44 PM CST on May 20th you would determine UT as follows:

1) For PM hours add 12-hours to get time in 24-hour format: 8:44 PM is 20:44 hours.

2) Add 6-hours to convert CST to UT: 20:44 + 6:00 = 26:44.

3) This exceeds 24-hours: add 1-day to calendar date and subtract 24-hours: 26:44 - 24 = 2:44.

4) The time is 2:44 UT the next day -- May 21st.


The celestial equator is a circle of reference created by an extension of the Earth's equator into space. For an observer standing on the equator, it would run dead east-west through the zenith -- the highest point in the sky. Observers at the poles would have the celestial equator running along the horizon. The ecliptic is another reference circle, created by using the plane of the Earth's orbit. The path taken by the sun across the sky traces a section of the ecliptic each day. The moon and planets move some degrees north or south of this circle. The ecliptic and celestial equator would be the same circle if the Earth's axis of rotation were perpendicular to its orbit. But, the planet is tilted: so these circles intersect each other at the two equinoxes and form an angle called the obliquity of the ecliptic. The vernal equinox is the intersection point that the sun reaches in spring and is used as the starting point for measuring angular distances along the ecliptic or equator.

Right ascension (RA) and declination (DEC) form the celestial equatorial system of measure, that uses the vernal equinox and celestial equator as starting points. It is similar to the system of longitude and latitude on the surface of the globe. RA is measured eastward along the celestial equator from the vernal equinox. It is given in units of hours, which correspond to 15 degrees of arc. In this way, 24 hours of RA equal 360 degrees of arc (24 x 15 = 360). These hours are subdivided into minutes and seconds, just like the hours on a clock (see the Mean Solar Day to Sidereal Day function at the end of Basic Conversions on why you can't use your watch to measure off RA). For purposes of calculation these hour:minute:second of position are first converted into degrees of arc. Declination is measured in degrees from the celestial equator (0-degrees) north (+) and south (-) to the celestial poles, which reside at +/-90 degrees of declination and coincide with the rotation axis of the planet.


Equatorial vs Ecliptic Coords The next most common positional system encountered by the amateur astronomer is celestial ecliptic. It shares with the RA and DEC system in the use of the vernal equinox as a starting point for positive eastward measurement. Measurement, though, is along or perpendicular to the circle of the ecliptic. The angular distance north or south of this baseline is ecliptic or celestial latitude. It runs up to 90-degrees ending at the ecliptic poles. Ecliptic or celestial longitude is the angular distance eastward along the ecliptic from the vernal equinox point. Unlike RA and DEC, both measurements are usually given in degrees.

Gravitational effects on the Earth, mainly from the Sun and Moon, cause the equinox points to shift along the celestial equator. A long term effect, known as precession, causes the celestial poles to rotate around the ecliptic poles in a cycle of 26,000 years. This has the unfortunate effect of allowing celestial coordinates for an object to change over time. Therefore, all such coordinates are given in terms of a date epoch. Currently the standard epoch is known as J2000.0. This is equivalent to noontime on the first day of the year 2000. You may also see epochs for quarter and midyears: 1991.25, 1999.5, etc., as well as instantaneous epochs.

Astro Utilities Electronic Book Copyright 1999 Pietro Carboni. All rights reserved.