... Known as an equinox, this astronomical event marks the first day of spring in the northern hemisphere and autumn in the south. ... Pictured above is the best multi-wavelength image yet of Tycho's supernova remnant, the result of a stellar explosion first recorded over 400 years ago by the famous astronomer Tycho Brahe. Martian Moon Deimos from MRO . ... Deimos is one of the smallest known moons in the Solar System measuring only about 15 kilometers across. ... Hickson Compact Group 90 . ...