Astronomy Picture of the Day . Unusual Spheres on Mars . ... Equinox: The Sun from Solstice to Solstice . ... Yesterday was an equinox, a date when day and night are equal. Today, and every day until the next equinox, the night will be longer than the day in Earth's northern hemisphere, and the day will be longer than the night in Earth's southern hemisphere. ... An equinox (equal night), this astronomical event marks the first day of autumn in the northern hemisphere and spring in the south. ...