Resources
Q & A
Glossary
Acronym Guide
Further Reading
Outside the Site
Google Sky
WWT
Facebook
Youtube
Vimeo
Twitter
Flickr
Pinterest
Multimedia, Etc
Images/Illustrations
Animation & Video
Special Features
Chandra Podcasts
Chandra Mobile
Desktop Images
The Big Chandra Picture
High Res Prints
Presentations
Handouts
Screen Savers
Audio
Web Shortcuts
Chandra Blog
RSS Feed
Chandra Mobile
Chronicle
Email Newsletter
News & Noteworthy
Image Use Policy
Questions & Answers
Glossary of Terms
Download Guide
Get Adobe Reader
Q&A: General Astronomy and Space Science

Q:
Why can't we see the autumn constellations in the night sky in the spring?

A:
You can generally think of star constellations as fixed in position and not moving -- the stars are so far away that over a human lifetime they don't really change position, for the most part. The Earth, however, does move -- it completes one orbit around the Sun every year. At night, we see the stars that are in the direction opposite the Sun. That is, at night, you're looking out into the universe in a particular direction -- and this direction changes as the Earth orbits the Sun:
                      E
* 
                                *
A* *      <-E1      Sun      E2->       * B *
*                                       *
                      E

So in one season the Earth is at E1, and at night you're looking toward constellation A (constellation B is overhead when during the day). In another season, the Earth is at E2, and at night you're looking toward constellation B (constellation A is then overhead during the day).

Back | Index | Next