The orbit of the Earth about the sun defines a plane. Earth's axis is tilted from the perpendicular of the plane by 23.5 degrees. This axial tilt means that the northern hemisphere tilts away from the sun in December, and toward the sun in June, creating the seasonal change over a year's time. When the Northern Hemisphere tilts toward the sun, the light rays strike the region more directly, at less of an angle from the perpendicular. It is the angle of the sun's rays striking the surface that bring about the greatest seasonal effect.

Earth's orbit is not a perfect circle, so its distance to the sun also changes during the year. It happens that the Earth is closer to the sun during December and farther from the sun during June, making the seasonal temperature fluctuation more drastic in the southern hemisphere than in the northern hemisphere. The variation of the distance between the Earth and Sun is a much smaller factor in creating seasonal changes.

The length of the time that the sun is above the horizon lengthens and shortens periodically. The shortest and longest days are called the winter and summer solstice, and the days that have equal amounts of day and night are called equinoxes.

Over the course of a year, earth takes a lap around the sun. At any given time of year, some stars will not be visible to us, because the lie behind the sun or are close enough to it that they are only “out” in the day. The apparent path that the sun takes around the celestial sphere over the course of one year is called the ecliptic, and the constellations that lie along the path that the sun appears to take in the sky are known as the zodiac stars. Ancient people such as the builders of Stonehenge and other early “observatories” were very adept at keeping track of the apparent motions of constellations, and developed calendars built on the knowledge that they gained by watching the heavens.

We define the time of day on Earth by the apparent position of the sun in the sky. When it is at its highest point in the sky, we say it is noon, and midnight is halfway between one noon and the next. A day is defined as the time interval from one midnight to the next. We call this a solar day. Again, it is not the motion of the sun that makes this happen, but rather, the rotation of the Earth.

A day as measured by one rotation of the Earth with respect to the background stars is a different amount of time. This is because the Earth moves around the sun while it is rotating, as exaggerated in the image above. The Earth has to rotate a bit farther for the sun to reach the same point in the sky as a background star. A day measured with respect to the background stars is called a sidereal day. A sidereal year is the time it takes for the Earth to complete on orbit around the sun, relative to the stars. A sidereal year is about 20 minutes longer than a tropical year, defined by the equinoxes and solstices.

Earth also undergoes a much longer cycle. Over the course of thousands of years, the axial tilt of the Earth precesses, much like a spinning top. This means that the North Star, Polaris, will not always be over the North Pole. The precession completes one cycle in about 26 thousand years.

This seems like a long time, but it is very short compared to the orbital period of the sun about the center of the Milky Way galaxy, which takes about 250 million years.

The moon orbits the Earth, which changes the relative positions of the Sun, Earth and Moon. This causes the phases of the Moon.

The side of the Moon that faces the Sun is always lit, but not always visible to us. When the Moon is directly between us and the Sun, we see its shadowed side, and call this view the "New Moon" phase. When the Earth is directly between the Moon and Sun, we see the compete lit side of the Moon, and call this the "Full Moon" phase.

Note that in the diagram above, there is a white line bisecting the Moon, indicating the half of the Moon that can be see from Earth.

When the lit side of the Moon just barely comes into view on Earth, the lit part of the Moon takes a thin crescent shape. As the lit portion is increasing, we say that the phase is "waxing" and as the lit portion is decreasing, we say that the phase is "waning." Just after the New Moon, the phase of the Moon is called the "Waxing Crescent" phase. The "First Quarter" and "Third Quarter" phases are actually when half of the Moon appears to be lit, in our view. When we see more than half of the lit surface of the moon, we call it a "Gibbous" phase. The Waxing Gibbous phase precedes the Full Moon, and the Waning Gibbous phase comes after the Full Moon.

Lunations are periods of orbit of Earth's moon. One lunation lasts approximately 29.5 Earth days. This animation was compiled from images taken by NASA's Lunar Reconnaissance Orbiter as it orbited the Moon. It shows the more subtle variations that take place over one lunation.

The Moon rotates once on its axis every time it completes one orbit. As you can see, the Moon seems to rock back and forth on its axis. This is because the Moon's rotational axis is tilted with respect to its orbital plane. This causes an effect much like the seasons on Earth. Sometimes the North pole of the Moon is tilted toward Earth, and sometimes away from Earth.

The Moon also appears to change size in the sky. This is because sometimes the Moon is closer to Earth than it is at other times. Its orbit elliptical, not a perfect circle.

Since the orbit is elliptical, the Moon travels faster when it is closer to Earth and slower when it is farther away. This causes a wobble called a libration, which makes the Moon appear to rock slightly from side to side.

We will study the effects of an elliptical orbit more closely when we address Kepler's laws in the Historical development section.

In the above diagram, consider that the Earth is rotating counterclockwise as the Moon is orbiting, also in a counterclockwise direction. The time of day is defined by the position of the Sun in the sky. Comparing this diagram with the diagram showing the Moon phases, you can see that the only time the First Quarter Moon can be directly overhead is at 6 pm.

A solar eclipse occurs when the Moon lies directly between the Earth and Sun, and a lunar eclipse occurs when the Earth lies directly between the Sun and Moon.

The fact that the orbital plane of the Moon is tilted with respect to Earth's orbital plane about the sun means that we do not see eclipses every time there is a New Moon or Full Moon phase.

A solar eclipse can be viewed using a filter to block the light of the sun, or indirectly, using a pinhole camera. Never look directly at a solar eclipse.

The path of the shadow of a solar eclipse can be predicted with great accuracy. This NASA video shows the predicted path of the shadow of the total solar eclipse of August 21, 2017.

This image shows the solar eclipse crossing Turkey, that occurred on March 18, 2015 as seen by the International Space Station.

A lunar eclipse is produced when Earth's shadow crosses the Moon. The reddened color arises through the sunsets and sunrises around the edges of the Earth.

This time lapse video of a lunar eclipse captures the whole event, as the shadow of the Earth passes over the moon.