First, two hard facts about the sky are that (1) changes in it happen slowly, and (2) we cannot cause it to change. We cannot turn it and look at it from a different angle; we cannot speed up the rotation of Earth or the motion of the planets through the constellations; we cannot cause an eclipse to happen; we cannot view the sky from another place on Earth (unless we actually go there); we cannot see what the sky looked like long ago or in the distant future. We are stuck in the here and now.
If the sky could speak, it might tell us, “Pay attention.” That is usually good advice, but it’s especially valuable when it comes to the heavens. The sky reminds us that things are not always as they seem.
It wasn’t too long ago that we humans believed Earth was the center of it all, that when we saw the Sun and stars parade across the sky, we were witnessing the universe revolving around us. It took the work of some bold and observant scientists, including the Polish astronomer Nicolaus Copernicus, to show us how wrong we had been: Instead of the center of the universe, Copernicus said, our world is one of many planets and moons circling the Sun in a celestial clockwork.
Some 250 years later the Apollo 8 astronauts became the first humans to see the truth of this with their own eyes. As they made history’s first voyage around the Moon, they looked back at Earth, whose apparent size diminished until they could cover it with an outstretched thumb. In that moment, they knew on a gut level that Copernicus was right. Most of us, though, have not had the privilege of such a view. Standing under a canopy of stars on a clear night, we’ve had to take on faith what astronomers have told us.
Rotation of the Sky
There is movement in the sky. Not only do the Sun, Moon, planets, comets, and asteroids move against the background of stars, but the sky itself moves. These changes happen because of the motions of Earth as it spins on its axis and orbits the Sun. As the night passes and as the seasons change, we face different parts of the universe and see different stars and constellations.
Many celestial motions are too slow to be noticed over so short a period as a night or even a month, but the nightly rotation of the sky happens on a scale that, with a little patience, we can experience while we gaze upward. The sky’s rotation is shown dramatically in long time-exposure photographs centered on the North Star, which show the motion of stars as circular trails of different sizes centered on the sky’s North Pole. We speak of the sky rotating overhead, although we know that it is Earth that is turning.
The Earth makes one rotation a day, spinning from west to east, which causes the sky to turn from east to west. We speak of the Sun rising in the morning, although we know that it is Earth that turns towards the Sun, making the Sun appear to rise above the horizon. The illusion is so convincing that it wasn’t until the time of Copernicus in the 16th century that people accepted that Earth does indeed turn on its axis.
The Earth appears to be at the center of everything when an observer gazes up at the sky. The Sun, Moon and stars rise in the east, cross the sky from east to west, and set in the west in a dance that repeats itself daily. Although it looks as if everything revolves around the Earth, the reverse is actually true. It is the Earth that rotates once every day underneath the sky. This general motion of all celestial objects is called diurnal motion; diurnal motion is motion that repeats itself every day.
Diurnal motion is an astronomical term referring to the apparent daily motion of stars around the Earth, or more precisely around the two celestial poles. It is caused by the Earth’s rotation on its axis, so every star apparently moves on a circle, that is called the diurnal circle. The time for one complete rotation is 23 hours, 56 minutes and 4.09 seconds (1 sidereal day). The first experimental demonstration of this motion was undertaken by Jean Bernard Léon Foucault.
Proving that Earth Spins
How would you demonstrate that Earth spins rather than the sky? No simple visual demonstration existed until the Frenchman Jean Bernard Léon Foucault hung a massive iron ball from the high dome of the Pantheon in Paris in 1851 and set it swinging. Foucault demonstrated that this pendulum appears to slowly change the direction of its swing relative to the ground beneath. Since the pendulum does not feel the orientation of the building it is attached to, Earth’s rotation does not affect the direction of its swing. The pendulum feels the sum of the gravitational pull of the rest of the universe and maintains a constant orientation relative to the distant stars. Foucault Pendulums are found today in planetariums and science museums.
Annual Motion of the Sun
The daily rotation of Earth on its axis is one fundamental motion of Earth (and of the sky). The second is the annual revolution of Earth around the Sun. Until the 16th century, it was taken as a matter of faith that Earth does not move and is the center of all creation. Ancient Greek musings contrary to this view were taken as mere philosophical speculations.
In 1543 the Polish astronomer Copernicus proposed that Earth orbits the Sun, rather than the other way around, but he had no proof of what was to him a mathematical issue. Two generations later the great Italian astronomer Galileo Galilei supplied this proof in the form of telescopic observations of the phases of Venus and the moons of Jupiter. He took up the “heliocentric” (Suncentered) cause, but ran afoul of the authorities for his methods.
The truth was out, however, and by the mid-1600s it was universally accepted (in Europe, at least) that Earth orbits the Sun. Long before anyone knew whether it was the Sun or Earth that moved, astronomers plotted the apparent path of the Sun in the sky, relative to the background stars. This path is known as the ecliptic.
Astronomers also noticed how the rise and set points of the Sun on the horizon and its noon-time elevation varied with the changing seasons. They even determined the length of the year – sometimes with surprising accuracy.
The Sun’s path among the stars has been considered special since it was first identified. The Sun moves through only certain constellations, and even in the earliest times these constellations were accorded extra importance. The Moon and planets pass through the same constellations, and this also contributed to their mystique.
The Moon stays close to the Sun’s path and provides a simple way to divide it into segments. In the time that it takes the Sun to travel around the sky once, the Moon crosses the sky just over 12 times (this is the long way of saying there are 12 months in one year). Rounding this to the convenient whole number 12 suggests that the Sun’s (and Moon’s) path be divided into 12 segments, each of an equal length (30°). Doing this links the motions of the Sun and Moon at least symbolically.
Along the Sun’s path are prominent groups of stars, like Scorpius and Gemini, and areas devoid of bright stars, like Aquarius and Cancer. The Sun passes through 13 of the 88 constellations mentioned earlier in its yearly journey through the stars. 12 of these 13 constellations are the classical constellations of the zodiac. They were all named by 600 BC, but most are far older. Scorpius, for example, has been seen as a scorpion for at least 6,000 years, which is long before the concept of the zodiac as a complete circle was worked out. Most people associate the zodiac with astrology.
Annual Changes in the Stars
The Sun’s apparent motion against the background of stars also causes seasonal changes to the constellations that we see at night. Each day the Sun is nearly 1° to the east, relative to the stars, of where it was the day before. If we think of the Sun as staying relatively still (and – after all – our timekeeping methods are based on the position of the Sun, rather than the stars), we can think of the stars as moving westward 1° per day relative to the Sun.
Stars rise four minutes earlier each day, or 1/2 hour earlier each week, or 2 hours earlier each month, or 24 hours earlier each year. This is another way of saying that the cycle has been completed and the stars rise at the same time again after one year has passed. If a star rises at 2 A.M. on one date, it will rise at midnight one month later, at 10 P.M. another month later, and at 8 P.M. yet another month later (some stars near the North Star are exceptions to this rule, for they are circumpolar, meaning that they do not rise and set, but remain above the horizon all day and night). This is a handy rule of thumb to remember when you are planning which stars and constellations to observe. If you have to stay up too late to see it now, wait a few months and it will be conveniently placed in the evening sky. This rule of thumb does not work for the Moon or for Mercury, Venus, and Mars, as they have their own motion against the stars. It is relatively accurate as a rule of thumb for Jupiter and Saturn (and the outermost telescopic planets) and for most asteroids, because they orbit the Sun very slowly and thus appear to share the same motion as the stars.
Of course, the stars also set four minutes earlier each day, 1/2 hour earlier each week, and 2 hours earlier each month. If Saturn or Orion sets at 8 P.M. this month, it will set at 6 P.M. next month – and you won’t see it. “What the Sun giveth, the Sun taketh away.”