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.
·
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. |