where you can find these bodies in our sky - and serve as hallmarks in our monthly or yearly observations of neighboring worlds. For example, when the moon is at eastern quadrature - or 90 degrees east of the sun - we say the moon is at the first quarter phase. At such times, the moon is at its highest in the sky at about 6 p.m. local time.
• And so it is with Jupiter at eastern quadrature. This bright planet is now approximately highest in the sky at 6 p.m. You can easily spot Jupiter because it is the brightest star-like object in the sky. From our northerly latitudes, Jupiter will set after the midnight hour local time.
• Less than three months ago - on December 3, 2012 - Jupiter was at opposition. It was opposite the sun in Earth's sky, or 180o from the sun. If you had looked down on the solar system at that time, you would have seen the sun, Earth and Jupiter making a straight line in space. At opposition, a heavenly body climbs highest in the sky at midnight.
• Opposition and quadrature can happen only to solar system bodies that orbit the sun outside of Earth's orbit. Planets that orbit the sun inside of Earth's orbit (Mercury and Venus) can never reach opposition or quadrature. Instead, they always remain near the sun as seen from Earth. So we see them either in the east before sunrise, or in the west after sunset.
• Jupiter's oppositions and quadratures enabled the innovative astronomer Copernicus (1473-1543) to compute Jupiter's distance from the sun. He did this by charting Jupiter's (and the Earth's) change of position from opposition to quadrature. All the while, Copernicus presumed that Jupiter and Earth both orbit a central sun.
• By using the Earth-sun distance as his baseline, Copernicus relied upon the magic of geometry to figure out that Jupiter is over five times the Earth's distance from the sun!
• So look for Jupiter as it stands at eastern quadrature - or 90 degrees east of the sun - on Monday, February 25.