Constellations and asterisms are often regarded as identical.
They are not. Up until modern times, constellation and asterism
meant the same thing – a recognizable group of stars, given a name indicating
an object seen in the stars’ pattern. All of that changed, in 1930,
when the International Astronomical Union defined the current 88 constellations.
Now, a constellation is a region in the sky , which, naturally,
contains stars. The entire celestial sphere has been divided into
88 regions, these regions being constellations. All stars, however
bright, dim, or invisible, belong to (are within) a constellation, even
if they contribute nothing to forming a coherent picture. Asterism
retains the original meaning – a group of stars, with a recognizable pattern.
The most striking difference between constellation and asterism (at least
in the northern hemisphere) is the difference between Ursa Major (a constellation)
and the Big Dipper (an asterism). Ursa Major is a region in the
sky that contains dozens of visible stars. The Big Dipper, however,
is just the seven bright stars that form the easily recognized pattern
of a bowl and handle (or scythe, or cart …).
Celestial Sphere – When viewing the stars from the surface of
Earth, we can visualize them as being points of light on a celestial sphere
surrounding the Earth. At night, with unobstructed horizons, we see
half of the celestial sphere. We define the locations of stars on
the celestial sphere via right ascension and declination. Right ascension
corresponds to longitude on the surface of the Earth, while declination
is the analog to latitude.
Great Circle – A circle within our celestial sphere than divides
it into two equal hemispheres. Distance along the circumference
of the circle is usually measured in degrees – 360 for the entire circle.
Celestial Equator – The great circle seen directly overhead at
the Earth’s terrestrial equator. It divides the celestial sphere into
its northern & southern hemispheres. It intersects due east and
due west at the observer’s horizon and is midway between the north &
south celestial poles.
Horizon – The great circle that divides the visible hemisphere
of the celestial sphere (above ground) from the invisible hemisphere (below
ground). It intersects the cardinal points (due north, east, south,
& west) and is midway between the zenith and nadir.
Zenith – The point on the celestial sphere directly above the
observer.
Nadir – The point on the celestial sphere directly beneath the
observer, opposite the zenith. Being underground, it is invisible.
Declination – The position of a star north or south of the celestial
equator, measured in degrees. The north celestial pole is +90 degrees.
The south celestial pole is –90 degrees. The celestial equator has
a declination of 0 degrees.
Right Ascension – The east/west position of a star, compared
to an arbitrary point on the celestial equator. This arbitrary point
is the vernal equinox, the point at which the sun crosses the celestial
equator (from south to north) approximately March 20th each year. One
would expect right ascension to be measured in degrees, but it is usually
measures in hours, minutes, and seconds. This is because change
in right ascension is used to indicate the passage of time. The entire
celestial equator is a 360 degree great circle, whose 0 degree point is
the vernal equinox. An hour of right ascension equals 15 degrees
(for each of the 24 hours in the day). A minute is 1/60th of an hour.
A second is 1/60th of a minute. A point 70 degrees to the east of
the vernal equinox would have a right ascension of 04h40m (4 hours, 40 minutes).
The autumnal equinox, at 180 degrees, is defined as 12h00m. The
sun’s summer solstice point (for the northern hemisphere) is approximately
90 degrees (06h00m); its winter solstice point is approximately 270 degrees
(18h00m).
Meridian – The imaginary great circle that intersects the
zenith, nadir, the north & south celestial poles, and due north &
south on the horizon. It divides the celestial sphere into eastern
& western hemispheres.
Ecliptic – The path of the sun through the stars during the year,
approximating a great circle. The celestial equator and ecliptic
are not equivalent. Because the Earth rotates around an axis that
is tilted approximately 23.5 degrees from its orbit around the sun, the
sun’s path only coincides with the equator at two points (the equinoxes).
The ecliptic is north of the equator from right ascensions 00h00m through
12h00m hours, when day is longer than night in the northern hemisphere.
The ecliptic is south of the equator from right ascensions 12h00m through
24h00m (or 00h00m), when night is longer than day in the northern hemisphere.
Magnitude – The brightness of a star. The lower the magnitude,
the brighter the star. The brightest star (excluding our sun),
Sirius, has a magnitude of approximately –1.45. When viewing conditions
are perfect, a person with keen vision can just barely see a star of magnitude
6.0.
Binary Stars – Many, if not most, stars are binary. The
two stars circle each other, but, because of their vast distance from us,
we see only one star with the naked eye. With telescopes or spectroscopy,
we can discern the multiple stars.
Variable Stars - Some stars (usually cyclically) change in magnitude.
One reason is a darker star eclipsing its brighter companion. The
most famous of these variable stars is Algol, named the Demon Star because
of this variability. Another reason is that the star contracts and
expands, such as the Cepheid variables. Mysteriously, Polaris is
defined as a Cepheid variable, but its magnitude has become more constant
over the past few decades. And some stars are simply unstable, such
as Eta Carina.
Precession of the Equinoxes – Within a single human being’s lifetime,
the vernal and autumnal equinoxes appear fixed. However, over the
span of millennia, the equinoxes are slowly moving. Approximately
2000 years ago, the vernal equinox was located between the constellations
of Aries and Pisces. Now, the equinox is in Pisces, and closer to
Aquarius than it is to Aries. In approximately 700 years, the vernal
equinox will move into Aquarius. Astronomers, who need precise measurements,
must account for this movement. Thus, right ascension must be defined
in terms of epoch (the vernal equinox for a particular year). On
this web site, I use right ascensions of epoch 2000 (00h00m of the vernal
equinox in the year 2000).
