Facebook introduces hashtags

Facebook is introducing hashtags, the number signs used on Twitter, Instagram and other services to identify topics being discussed and allow users to search for them.

Venus invades the Pleiades

This week the second planet from the sun will pass directly in front of the Pleiades star cluster. It's a rare sunset conjunction that's easy to find with the unaided eye, but best seen through binoculars or a small telescope.

See Mercury at sunset

The timing couldn't be better. Just as NASA's MESSENGER probe is about to enter orbit around Mercury, the innermost planet is popping out of the twilight for its best apparition of 2011.

Perseid Meteor Shower Peaks Tuesday, Aug. 12

(PhysOrg.com) -- It's time once again for nature to put on its late-summer fireworks show: the Perseid meteor shower. This year's best viewing will be before dawn on Aug. 12, with a second chance after sunset that night, ...

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