Explanation of the Motion of the Earth by Ron Kurtus - Succeed in Understanding Astronomy. Key words: Physics, Physcial Science Sun, galaxy, Milky Way, rotation, orbit, seasons, ellipse, axis, daylight, hemisphere, leap year, School for Champions. Copyright © Restrictions

# Motion of the Earth

by Ron Kurtus (revised 18 June 2010)

One of the important characteristics of the Earth is its movement. Seeing the Sun move through the sky is a result of the fact that the Earth rotates on its axis. As with all planets in our Solar System, the Earth orbits or moves around the Sun. The Earth's axis is slightly tilted with respect to its orbit around the Sun, resulting in the change of seasons. The Earth also follows the Sun in its motion through space. The various motions result in days, seasons and years.

Questions you may have include:

• How long is a single rotation of the Earth on its axis?
• What factors does revolving around the Sun affect?
• What other motions does the Earth have?

This lesson will answer those questions.

## Rotates on its axis

The Earth is a sphere that rotates, on its axis, passing through the North and South Poles. The rotation is in a counterclockwise direction, looking down at the North Pole.

The time it takes for the Earth to make a complete rotation is approximately 24 hours (exactly 23.934 hours). This rotation results in daytime when an area is facing the Sun and nighttime when an area is facing away from the Sun.

Earth rotates on its axis

Since we are on Earth, we do not sense its rotation, but we experience it by observing the relative motion of the Sun. It is similar to what you experience when you are sitting in a moving automobile and see the surroundings move by.

## Revolves around the Sun

The Earth revolves around the Sun, in a counterclockwise direction, once every 365.25 days. Its velocity in space in its orbit is about 18.5 miles per second.

The shape of the orbit is a slight ellipse. Seldom—if ever—is an astronomical orbit a true circle.

Earth rotates in an elliptical orbit around the Sun

### Leap years

Since a year has been designated as 365 days, a day is added every 4 years to even things out. That year is called a "Leap Year" and occurs on years divisible by 4, such as 2004. The extra day in a leap year is added in February.

Actually, the period is slightly less than 365.25 days. Thus a leap year is any year divisible by 4, except for years that are both divisible by 100 and not divisible by 400. Therefore, the year 2000 was a leap year, but the years 1700, 1800 and 1900 were not. That is complicated, but then again, who wants things easy?

### Seasons

The axis of the Earth is on a slight tilt with respect to the orbit around the Sun. That tilt is always in the same direction and results in the changing seasons on the Earth.

Tilt of the Earth is in the same direction

The reason the seasons change has to do with how direct the Sun is shining, as well as the length of a day.

#### Summer

When the Earth is in the part of the orbit around the Sun where the sunlight shines more directly on the Earth, the days are longer and less sunlight is reflected. That is summer, and the weather becomes warmer.

Sunlight shines more directly in summer

Due to the spherical shape of the Earth, there is more daylight in the summer the closer you get to the pole. One you cross the Arctic Circle in the Northern Hemisphere in summer or the Antarctic Circle in the summer in the Southern Hemisphere, a day can be 24 hours long. That is why they can it "the land of the midnight Sun."

#### Winter

On the other side of the orbit around the Sun, the angle of the sunlight is steeper, resulting in more light being reflected. Also, the days are shorter. This results in the colder weather of winter.

Sunlight reflects off steeper angle in winter

Since the orbit of the Earth around the Sun is an ellipse, surprisingly, the Earth is closer to the Sun for a Northern Hemisphere winter than it is in the summer. That would imply that the Southern Hemisphere has slightly colder winters and warmer summers than the Northern.

## Other motion

Besides spinning on its axis and revolving around the Sun, the Earth also follows the Sun's movement through the Milky Way galaxy. The Sun and its planets are moving toward the star Vega at a velocity of about 12 miles per second. The Sun and Vega are rotating about the center of the Milky Way at about 150 miles per second.

The Milky Way—in turn—is moving through space, as part of the apparent expansion of the Universe. Its estimate velocity is thousands of miles per second depending on which galaxy you compare it.

Since the distances are so great, we will never notice these motions with respect to the other stars and galaxies in the immense Universe.

## Summary

The Earth spins on its axis and revolves around the Sun. The tilt of its axis with respect to the Sun causes the Earth's seasons. The Earth also follows the Sun's rotation around the center of the Milky Way galaxy, which is moving through space.

Be honest and honorable

## Resources and references

Ron Kurtus' Credentials

### Websites

Earth Facts and Figures - From NASA

The Earth - Details from Nine-Planets website

Space Weather - News about Earth-Sun environment

Astronomy Resources

## Questions and comments

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