Earth's Orbit
Earth orbits the Sun at an average distance of 149.60 million km (92.96 million mi), and one complete orbit takes
365.256 days (1 sidereal year), during which time Earth has traveled 940 million km (584 million mi). Ignoring the
influence of other solar system bodies, Earth's orbit is an ellipse with the Earth-Sun barycenter as one focus and a
current eccentricity of 0.0167; since this value is close to zero, the center of the orbit is close, relative to the
size of the orbit, to the center of the Sun.
As seen from Earth, the planet's orbital prograde motion makes the Sun appear to move with respect to other stars at
a rate of about 1° eastward per solar day (or a Sun or Moon diameter every 12 hours).[nb 1] Earth's orbital speed
averages 29.78 km/s (107,208 km/h; 66,616 mph), which is fast enough to cover the planet's diameter in 7 minutes and
the distance to the Moon in 4 hours.
From a vantage point above the north pole of either the Sun or Earth, Earth would appear to revolve in a
counterclockwise direction around the Sun. From the same vantage point, both the Earth and the Sun would appear
to rotate also in a counterclockwise direction about their respective axes.
Events in the Orbit
By astronomical convention, the four seasons are determined by the solstices (the two points in the Earth's orbit of the maximum tilt of the Earth's axis, toward the Sun or away from the Sun) and the equinoxes (the two points in the Earth's orbit where the Earth's tilted axis and an imaginary line drawn from the Earth to the Sun are exactly perpendicular to one another). The solstices and equinoxes divide the year up into four approximately equal parts. In the northern hemisphere winter solstice occurs on or about December 21; summer solstice is near June 21; spring equinox is around March 20; and autumnal equinox is about September 23.[7] The effect of the Earth's axial tilt in the southern hemisphere is the opposite of that in the northern hemisphere, thus the seasons of the solstices and equinoxes in the southern hemisphere are the reverse of those in the northern hemisphere (e.g. the northern summer solstice is at the same time as the southern winter solstice).In modern times, Earth's perihelion occurs around January 3, and the aphelion around July 4 (for other eras, see precession and Milankovitch cycles). The changing Earth–Sun distance results in an increase of about 6.9% [8] in total solar energy reaching the Earth at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that the Earth reaches the closest approach to the Sun, the southern hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. However, this effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the southern hemisphere.[9]
The Hill sphere (gravitational sphere of influence) of the Earth is about 1,500,000 kilometers (0.01 AU) in radius, or approximately four times the average distance to the Moon.[10][nb 2] This is the maximal distance at which the Earth's gravitational influence is stronger than the more distant Sun and planets. Objects orbiting the Earth must be within this radius, otherwise they may become unbound by the gravitational perturbation of the Sun.
Tilt of the Seasons
During the year, the seasons change depending on the amount of sunlight reaching the Earth as it revolves around the Sun.
The seasons are caused as the Earth, tilted on its axis, travels in a loop around the Sun each year. Summer happens
in the hemisphere tilted towards the Sun, and winter happens in the hemisphere tilted away from the Sun. As the Earth
travels around the Sun, the hemisphere that is tilted towards or away from the Sun changes.
The hemisphere that is tilted towards the Sun is warmer because sunlight travels more directly to the Earth’s surface
so less gets scattered in the atmosphere. That means that when it is summer in the Northern Hemisphere, it is winter in
the Southern Hemisphere. The hemisphere tilted towards the Sun has longer days and shorter nights. That’s why days are
longer during the summer than during the winter.
In general, the further away from the equator you travel, the cooler summer and winter temperatures become. At the
equator there are no seasons because each day the Sun strikes at about the same angle. Every day of the year the
equator receives about 12 hours of sunlight. The poles remain cool because they are never tilted in a direct path
of sunlight. Much light is scattered by the atmosphere before reaching the Earth surface at the poles.
During midwinter, when a pole is tilted away from the Sun, there is no daylight at all. The sun never rises!
However, during the summer, a pole receives sunlight all the time and there is no night!
Why Do Most Stars and Constellations Move?
The stars are distant objects. Their distances vary, but they are all very far away. Excluding our Sun, the nearest
star, Proxima Centauri, is more than 4 light years away. As Earth spins on its axis, we, as Earth-bound observers,
spin past this background of distant stars. As Earth spins, the stars appear to move across our night sky from east
to west, for the same reason that our Sun appears to “rise” in the east and “set” in the west.
Stars close to the celestial poles, the imaginary points where Earth’s north and south axes point in space, have
a very small circle of spin. So if you find Polaris, Earth’s north “pole star,” you will observe it move very,
very, very little in the night sky. The farther from Polaris, the wider the circle the stars trace. Stars that
make a full circle around a celestial pole, like those in the Big and Little Dippers in the northern hemisphere,
are called “circumpolar stars.” They stay in the night sky and do not set. At the equator, there are no
circumpolar stars because the celestial poles are located at the horizon. All stars observed at the equator rise
in the east and set in the west.