## Shop Windows to the Universe

Our Glaciers: Then and Now activity kit helps you see the changes taking place in glaciers around the world. See all our activity kits and classroom activities.

This is a picture taken from the Atlantis shuttle. It shows the sun reflecting off of the ocean waters of Earth.
Click on image for full size
Courtesy of NASA

# What is the distance from the Earth to the Sun when the Earth is at perihelion and when it is at aphelion? At what month is the Earth closest to the Sun? What is the circumference of the Earth? How fast is the Earth moving about its axis; how about around the Sun?

#### Ok. Let's get some terminology down first. If a body (like the Earth) is orbiting around the Sun, we say it is closest to the Sun at perihelion and farthest from the Sun at aphelion. If a body (like the Moon) is orbiting around the Earth, its closest point to the Earth is perigee and its farthest point from the Earth is apogee. In 2000, perihelion for the Earth was on January 3, 2000, and aphelion was on July 4, 2000. The Earth was 91,405,436 miles from Sun at perihelion and 94,511,989 miles from Sun at aphelion. For the year 2001, perihelion will occur on January 3, 2001 and aphelion will occur on July 4, 2001. The actual date for perihelion and aphelion will differ from year to year. But, you can see that the Earth is closest to the Sun in January and farthest from the Sun in July! This may not seem right. I mean it's winter in the northern hemisphere in January and yet we are closest to the Sun. But, our seasons are actually determined by the tilt of the Earth and not the Earth's proximity to the Sun.A few more numbers for you...the circumference of the Earth is 24,901.55 miles (40,075.16 km) at the equator. If you were to measure from the north pole to the south pole and back to the north pole, the circumference would be a bit shorter. That is because the Earth bulges a little bit around its waist (the equator). This "equatorial bulge" is caused by the Earth's spin - try spinning a water balloon as you toss it up into the air to see how this works. Tidal forces caused by the pull of gravity from the Sun and Moon also add a little to this bulge. So the pole to pole circumference is 24,859.82 miles (40,008 km). Finally, the gravitational pull between the Earth and Sun causes the Earth to travel around, or "orbit", the Sun at a velocity of 29.8 km/sec. At the same time, the Earth also turns on its axis causing the daily cycle of day and night. This "rotational velocity" is approximately .47 km/sec. That means that at the same time we're hurtling through space at nearly 67,000 mph, we're also spinning around in circles at over 1000 mph! Whew! Almost makes you dizzy, doesn't it?

Submitted by Leslie (Missouri, USA), Wayne (New York, USA), Terry (Michigan, USA), Kristi (Toronto, Canada), Wykeenie (Louisiana, USA), Tommy (Pennsylvania, USA)
(November 7, 2000)

#### Shop Windows to the Universe Science Store!

Learn about Earth and space science, and have fun while doing it! The games section of our online store includes a climate change card game and the Traveling Nitrogen game!

## Science, Evolution, and Creationism

How did life evolve on Earth? The answer to this question can help us understand our past and prepare for our future. Although evolution provides credible and reliable answers, polls show that many people turn away from science, seeking other explanations with which they are more comfortable....more

## The Seasonal Merry-Go-Round

The tilt of Earth's rotational axis and the Earth's orbit around the Sun work together to create the seasons. As the Earth travels around the Sun, it remains tipped over in the same direction, with its...more

## The Earth's Orbit

Like all planets in our solar system, the Earth is in an elliptical orbit around our Sun. In Earth's case, its orbit is nearly circular, so that the difference between Earth's farthest point from the Sun...more

## The Earth's Rotation

The Earth is rotating around an axis (called its rotational axis). Some objects rotate about a horizontal axis, like a rolling log. Some objects, such as a skater, rotate about a vertical axis. The Earth's...more

## Me and my boyfriend are arguing over whether or not the Moon is round (circular like Earth). I say it is and he says it's not.

What is the diameter of the Moon in Kilometers? By how much is the Earth heavier than the Moon? How far is the Moon from the Earth? How old is the Moon? What is the internal structure of the Moon? I was...more

## What Role Have Women Played in the History of the Space Program

*Please note that this page is a student project written by Nicole Turner. It was not written or edited by Windows to the Universe scientists.* From Harriet Quimby (the first licensed woman pilot,) to...more

## What are the most commonly used symbols for the planets, the Sun, and the Moon? What is the symbol for Venus?

Here they are! Submitted by Ardith and Ashley (January 26, 1999) ...more

## Does wind have an effect on radio waves?

Wind does not have an effect on radio waves. Radio waves have a rather long wavelength, from 10 to 100 meters. Wind cannot affect radio waves because the air particles associated with wind are far too...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information.