How about a triple conjunction? It's not something from an old grammar lesson. It's a triangle of bright points in the sky right after sunset each day this weekend.

Mercury, Venus and Jupiter are clustering together to form a triangle from our vantage point. Especially on Sunday night, when they will be less than 1.5 degrees apart, you can cover all three with your thumb held at arm's length.

Start looking each night after 8:30 in the low northwest sky. Venus will appear brightest. Binoculars will be helpful for spotting all three - but astronomers say, with clear skies forecast, we should be able to observe the triangle of planets with the naked eye. Be quick - it won't take long each night for them to start slipping below the horizon.

The next triple conjunction isn't coming until October of 2015. But you'll have to wait until 2026 to see three planets in such a close formation.

Here's more from National Geographic.