Astronomy Picture of the Day . Orion Nebula in Surrounding Dust . ... What surrounds a hotbed of star formation? ... If you see a sky like this -- photograph it. ... Today is the equinox. ... When does the line between day and night become vertical? ... Tomorrow is an equinox on planet Earth, a time of year when day and night are most nearly equal. At an equinox, the Earth's terminator -- the dividing line between day and night -- becomes vertical and connects the north and south poles. ...