## Shop Windows to the Universe

Science, Evolution, and Creationism, by the National Academies, focuses on teaching evolution in today's classrooms. Check out the other publications in our online store.

# How long does it take for Mars to rotate on its axis? What is the periodof the Earth? The Moon?

#### Webster's defines a "period" as "the completion of a cycle, a series of events, or a single action". With reference to the motion of planets and moons, a period can be defined in a couple of different ways. The "rotational period" of a celestial body is the amount of time required for it to complete one turn, or rotate on its axis. On Earth, we call this cycle a "day". Earth's day is exactly 23.9345 hours (23 hours, 56 minutes, 4.2 seconds). Mars' day is a little longer, at 24.6229 hours. The Moon is much slower, it takes 655.728 hours (27.322 days!) to complete one turn on its axis. In addition to rotational movement, the bodies in our solar system also move around, or orbit, other bodies. Planets orbit the Sun while moons orbit their respective planets. The amount of time required for a body to complete one orbit around another body is therefore referred to as an "orbital period", also called a "revolutionary period". On Earth, we call this orbital period a "year". Our year is 365.256 days. On Mars you'll get a lot more time between birthdays since it takes 686.98 days (Earth days, that is) to complete one orbit about the Sun. Since the Moon's orbit centers on the Earth, rather than the Sun, it would be misleading to think of the Moon's orbit as a "year". Instead, this cycle of the Moon orbiting the Earth more closely approximates our system of calendar "months". It takes 27.322 days for the Moon to orbit the Earth. If that number looks familiar, it's because the orbital period of the Moon is exactly the same as its rotational period. This is the reason we can never see the far side of the Moon from Earth--it's always facing away from us.

Submitted by Elizabeth (West Virginia, USA)
(October 15, 1997)

#### 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!

## What are the retrograde motions of planets in the sky?

It depends on what kind of motion you are talking about. When seen from the north pole of the celestial sphere all planets orbit around the Sun in a counter-clockwise or direct path. Most planets also...more

## How do Astronauts Live in Space?

Almost everyone has a question or two about living in space. What do astronauts do in space? How do they do everyday things like eat, sleep and go to the bathroom? Well, this is our attempt to answer...more

## How far is the Earth from the Sun, the Moon and all the other planets? How far are all of the planets from the Sun? Do you know of a software that tracks the planets in real-time?

There is a really neat internet program called Solar System Live that shows the position of all of the planets and the Sun for any given day. If you go to that page, you'll see an image similar to the...more

## Is it really true that man never really walked on the Moon?

The picture of the American Flag (the one put there by the Apollo astronauts) is waving (or straight out) in the wind. How could that be possible if there is no atmosphere on the Moon? Was it some sort...more

## How many planets orbit the sun?

I was wondering if there is a new planet? Are there planets (a tenth planet?) after Pluto belonging to our solar system? What are the names of the new planets discovered in the solar system? Are there...more

## According to Stephen Hawking, any object with an energy which equals Plank's energy has to become a black hole.

If that is so, the energy released during the Big Bang must have created many such black holes. Therefore most of the Energy of the Big bang must have disappeared in that form. Then how did the Universe...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.