Are Earth and the Moon "twin planets"?
Is the Moon a satellite of Earth or are Earth and the Moon a twin planet system?
I would say both. Whenever a group of bodies sharing an orbit the Sun also orbit each other, the largest one is called the planet and the rest are considered satellites. So, theoretically, if you have a double planet where one component is only 1% larger than the other, we would still call them "planet" and "satellite".
Definition of a double planet is less clear. There is no obvious lower limit to how big (or massive) a satellite has to be relative to the planet in order for the system to be considered a "double planet". Pluto and Charon (1/10 of Pluto's mass) are definitely a double planet and most people would consider Earth and the Moon (1/81 of Earth's mass) a double planet.
It is possible that once some Earth-sized extrasolar planets with moons are discovered, the term "double planet" will be defined more strictly, but so far it is completely unofficial.
Get More 'Curious?' with Our New PODCAST:
- Podcast? Subscribe? Tell me about the Ask an Astronomer Podcast
- Subscribe to our Podcast | Listen to our current Episode
- Cool! But I can't now. Send me a quick reminder now for later.
- How close was the Moon to the Earth when it formed?
- The Moon and Sun are the same apparent size in the sky. The Moon is moving away from us at 3.8 centimeters/year. Isn't it improbable that we live in an epoch in which they appear the same size in the sky?
How to ask a question:
If you have a follow-up question concerning the above subject, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.Table 'curious.Referrers' doesn't existTable 'curious.Referrers' doesn't exist
This page has been accessed 33528 times since January 12, 2003.
Last modified: January 13, 2003 11:53:12 PM
Ask an Astronomer is hosted by the Astronomy Department at Cornell University and is produced with PHP and MySQL.
Warning: Your browser is misbehaving! This page might look ugly. (Details)