Constellations are groups of stars. In 1928, the International Astronomical Union (IAU) established 88 official constellations with carefully defined boundaries that together include every part of the sky. The sky also contains a number of less formally defined groupings called asterisms, such as the Big Dipper and the Great Square of Pegasus. Constellation names came from Greek translated into Latin. Most individual star names come from Arabic. Circumpolar constellations never set.
A useful way to identify stars was invented by Johann Bayer in a catalog published in 1603. Bayer assigned Greek letters to the bright stars in each constellation in an order determined by a combination of brightness and position.
Astronomers describe the brightness of stars using the magnitude scale; many historians attribute its invention to Hipparchus. Early astronomers divided the stars into six classes (magnitudes). The larger the magnitude number, the fainter the star. Modern astronomers can measure the brightness of stars to high precision; some stars are actually brighter than magnitude 1.0 and the modern scale extends into negative numbers. The faintest stars you can see with your unaided eyes is about sixth magnitude. These numbers are known as apparent magnitude. Apparent magnitude doesn’t take into account radiation that human eyes can’t see or the distance to the star. In other words, apparent magnitude tells you only how bright the star looks as seen from Earth, not about its actual light output.
Flux refers to a measure of light energy from a star. The magnitude scale has been defined so that two stars differing in brightness by 5 magnitudes have a flux ratio of exactly 100. Therefore, in the case of two stars that differ by 1 magnitude, the light from one star is 2.51 times brighter than the other. Raising 2.51 to the power of the magnitude difference between two stars tells you how many times one star is brighter than the other.
The eastward motion of Earth causes the Sun, Moon, planets and stars to move westward in the sky as if the celestial sphere were rotating westward around Earth. The zenith marks the point of the celestial sphere directly above your head. The nadir marks the point of the celestial sphere directly under your feet. The apparent pivot points are the north and south celestial poles. Halfway between the celestial poles lies the celestial equator. The angular distance between two objects in the sky is the angle between two lines extending from your eye to the two objects. Astronomers measure angles in degrees and minutes and seconds of arc. An arc minute is 1/60th of a degree and an arc second is 1/60th of an arc minute. The angular diameter of an object is the angular distance from one edge to the other. The angular distance from the horizon to the (north or south) celestial pole is equal to your latitude (north or south).
Precession is the change in the orientation of the axis of rotation of the Earth; Earth spins like a giant top, caused by the gravitational forces of the Moon and Sun acting on the equatorial bulge of the spinning Earth. The precession of Earth’s axis takes about 26,000 years for one cycle. As a result, the positions of the celestial poles and celestial equator move slowly against the background stars.
Rotation is the turning of a body on its axis, whereas revolution means the motion of a body around a point outside the body. Earth’s motion in its orbit causes the Sun to move eastward against the background stars between sunrise and sunset a distance roughly equal to its diameter (and approximately 1 degree, about twice its angular diameter, in 24 hours). The ecliptic is the apparent path of the Sun against the background stars.
Earth’s axis is tipped 23.4 degrees from vertical, resulting in the seasons, caused by the angle of sunlight and how long the Sun is in the sky at different times of the year. The beginnings of the seasons are marked by the vernal equinox, the summer solstice, the autumnal equinox, and the winter solstice. Perihelion and aphelion mark the nearest and farthest points respectively of a body’s orbit around the Sun. Earth is 1.7 percent closer than the average to the Sun in January and 1.7 percent farther than the average to the Sun in July. This change in distance to the Sun has almost no effect on Earth’s seasons.
All of the planets of our Solar System move in nearly circular orbits around the Sun. Seen from Earth, the outer planets move slowly eastward along the ecliptic. The inner planets also stay near the ecliptic but they are never seen far from the Sun in the sky. The zodiac is a band around the sky extending about 9 degrees above and below the ecliptic in which the Sun, Moon, and planets are always found. The signs of the zodiac take their names from the 12 principal constellations along the ecliptic.
The Milankovitch hypothesis states that small changes in the shape of Earth’s orbit and axis inclination, along with a subtle effect of precession, could combine to influence Earth’s climate and cause ice ages. The most recent ice age began about 3 million years ago and is still going on. Between ice ages, Earth is warmer and there are no ice sheets even at the poles. Ice ages last from tens of millions to hundreds of millions of years.
—January 2021
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Lance Jones 