The sky — astronomy

Earth as a Planet

Earth is the third planet from the Sun. It spins once in about 24 hours and circles the Sun in about 365.25 days. Its axis is tilted by roughly 23.4°, which gives us the seasons and makes the sky look different from different latitudes.

Earth in the Solar System

Earth is the third planet from the Sun and the largest of the four rocky planets in the inner Solar System. Its average distance from the Sun is about 150 million kilometres, a length called the astronomical unit (AU), which astronomers use as a yardstick across the Solar System. Earth's mean diameter is about 12,740 kilometres. So far it is the only place we know for certain to harbour life.

Why is Earth round?

Gravity pulls matter toward a common centre. In a body as massive as a planet this pull is strong enough to smooth out any large bumps, so the body settles into a nearly spherical shape. Earth is not a perfect sphere: because it spins, it is slightly flattened at the poles and bulges at the equator. The difference between the equatorial and polar diameters is about 43 kilometres, a little over 0.3 percent. The best description is an oblate spheroid.

Day and year

Rotation

Earth spins around its axis from west to east. That is why the Sun, Moon and stars appear to rise in the east and set in the west. In reality it is we who move, not the sky. At the equator the surface moves at roughly 1,670 kilometres per hour, while near the poles the speed approaches zero.

One full turn relative to the distant stars, called a sidereal day, takes about 23 hours 56 minutes. A solar day, from one noon to the next, averages 24 hours. The difference of about four minutes arises because Earth also moves along its orbit, so it must turn a little extra to bring the Sun back to the same position. As a result a given star rises about four minutes earlier each night, which adds up to a full day over a year. This is why the evening stars change through the seasons. For more on this, see sidereal time.

Orbit

Earth circles the Sun in about 365.25 days at roughly 30 kilometres per second. The extra quarter of a day is why we add a leap day every four years. The orbit is nearly circular but slightly elliptical: Earth is closest to the Sun in early January (about 147 million kilometres) and farthest in early July (about 152 million kilometres).

From Earth, the Sun appears to travel through the year against the background stars along a path called the ecliptic, moving about one degree per day.

Axial tilt and the seasons

Earth's axis is tilted by about 23.4° from the perpendicular to its orbital plane. This tilt, not the distance from the Sun, causes the seasons. The proof is that the Northern Hemisphere has summer when Earth is near its farthest point from the Sun.

When the Northern Hemisphere leans toward the Sun, sunlight arrives more directly and days are longer, so it is summer there. At the same time the Southern Hemisphere leans away and has winter. Half a year later the roles swap. The moments of greatest lean are the solstices, and when neither hemisphere leans toward the Sun we have the equinoxes. To read more about the year's rhythm, see The Seasons.

The tilt direction is not fixed forever. Like a spinning top, Earth's axis slowly traces a cone and points toward different parts of the sky, completing one cycle in roughly 26,000 years. This is called precession. Because of it, today's Pole Star has not always been, and will not always remain, the star nearest the celestial pole.

The sky from different latitudes

Your latitude decides which stars you see and how they move. The height of the celestial pole above your horizon equals your latitude.

• At the North Pole the celestial pole is directly overhead, with Polaris very close to it. Stars circle parallel to the horizon and none rises or sets. The Sun stays above the horizon for about half a year, then below it.
• At mid-northern latitudes (for example Bulgaria, around 42 to 44° N) Polaris stands about 42 to 44° above the northern horizon. Stars near the pole are circumpolar: they never set. Others rise and set at a slant.
• At the equator both celestial poles lie on the horizon. Stars rise and set almost vertically, and over a year nearly the whole sky of both hemispheres is visible. Day and night stay roughly equal all year.
• In the Southern Hemisphere Polaris is not visible, but objects such as the Southern Cross appear, which are out of reach from northern latitudes.

The same geometry applies to the Sun. At noon its altitude is about 90° minus your latitude plus the Sun's declination (up to about ±23.4°). That is why the Sun can pass directly overhead only between the Tropics, and why beyond the polar circles, near 66.6° latitude, there are days of polar day and polar night.

In short

Rotation gives us day and night and the apparent motion of the sky. The orbit gives us the year and the changing constellations. The tilt gives us the seasons and the varying length of daylight. Where we stand decides which part of this sky we can see. Once you understand these three motions, the sky stops being a mystery and becomes a map you can read.

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