Brought to you by Starry Night

Saturday, June 10

We may be 10 days away from the first day of summer, but no doubt most of you are in summer mode already. The night sky is too.

Last month at 10 o'clock, the sky was darker and the spring constellations still prevailed. Tonight, they begin their exit "stage west," making way for the summer stars that are even now assuming dominance in the eastern evening sky.

Our two maps show the sky as it looks tonight (map A) and last month (map B). Notice how much higher in the sky Leo was last month than it is tonight.

Each month, the stars rise (or set) two hours earlier than the previous month. This amounts to an angular shift of 30 degrees to the east. Thirty degrees on the sky is roughly equivalent to 60 full moon-widths or just a little longer than the Leo pattern from Regulus to the lion's tail star, Denebola.

Looking at Leo tonight, you can pretty well estimate where it's going to be this time next month. The lion's tail will be where his head is, while his head will be less than 10 degrees above the western horizon. After that, we'll have to say, "So long Leo. See you next March."

Tonight's moon phase.

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Jeff Kanipe is the author of A Skywatcher's Year, an astronomy guide just published by Cambridge University Press. He is a former editor at Astronomy and StarDate magazines and a writer for the Earth & Sky radio series.

The images in Skywatch are produced by Starry Night software.