By Jim McCausland, Sunset senior science nerd

Now is a good time to plant fall and winter vegetables. Just be careful about where you sow them, since garden beds that get plenty of sunshine now may be shaded in winter, when the track of the sun is much lower in the sky. But how would you know where those winter shadows will fall? This month’s full moon (Friday, July 18) tells all.

Thursday evening, set your alarm for 1 a.m. (If this plan won’t fly with your spouse, drink a couple of glasses of water before you go to bed, and you’ll be up at the appropriate time.) Go outside when the alarm goes off, and you’ll find the full moon floating in almost exactly the same spot the sun will occupy five months from now. Whatever beds get moonlight now will get sunlight then. Whatever beds are shaded by buildings and evergreen trees now will be shaded by the same things in December.

This phenomenon works because the summer sun and the winter moon both rise at about the same place in the northeast, follow the same high track across the sky, and set in the northwest. The winter sun and the summer moon rise in the southwest, cross low in the sky, and set in the southwest. Around autumn equinox, both rise at about the same place in the east and set in the same place in the west (that’s why eclipses commonly happen in spring and fall).

The difference between the sun’s summer and winter tracks is sharp. Today in Seattle, the midday sun is 64° above the horizon. In  late December, it will be only 19° above the horizon. In Los Angeles today, the midday sun is 77° above the horizon, while in December, it will be 33° above the horizon. The difference is what causes such long winter shadows.