Photo top of post: Duke Marsh
The half-lit first quarter moon is found low in the southern sky at dusk on Friday, October 11. Or if you live in the Southern Hemisphere, you’ll find the moon high overhead after sunset. Meanwhile, the planet Jupiter is now rising in the middle of the night, to appear at its highest around dawn. Around the world tonight, the moon and Jupiter stand more or less opposite (180o) of each other on the sky’s dome. So when the moon sets in the west-southwest sky late tonight, you can expect the planet Jupiter to rise in the east-northeast sky in rather close conjunction with moonset.
The moon reaches its first quarter phase on October 11, 2013, at precisely 23:02 Universal Time. Translating Universal Time to the clock times in the United States, that places the time of tonight’s first quarter moon at 7:02 p.m. EDT, 6:02 p.m. CDT, 5:02 p.m. MDT or 4:02 p.m. PDT.
At first quarter, the moon is also said to be at eastern quadrature, or 90o east of the setting sun. By coincidence, the planet Jupiter reaches western quadrature, or lies 90o west of the rising sun tomorrow, on October 12. The moon has a western quadrature (90o west of the rising sun), too, by the way, at its last quarter phase.
Like the last quarter moon, a planet at west quadrature is seen in the morning sky, usually in the hours between midnight and sunrise. If you’re up during the predawn and dawn hours, it’ll be hard to miss Jupiter, the brightest star-like object in the morning sky.
Unlike the last quarter moon, Jupiter’s disk won’t look half-lit through the telescope. From our vantage point in the inner solar system, far-off Jupiter always looks full from Earth. However, as viewed from our planet, the shadows of Jupiter and its four major moons – Io, Europa, Ganymede and Callisto – angle out at a maximum of 12 degrees westward from these worlds at west quadrature.
That’s why, at and near west quadrature, the shadows of Jupiter’s moons cross Jupiter’s disk a maximum amount of time before the moons themselves do. As the moons circle behind Jupiter, the moons sweep into Jupiter’s long shadow a maximum time before the moons swing behind the giant planet itself.
Nearly three months from now, on January 5, 2014, Jupiter will be at opposition – 180o – from the sun in our sky. At and near opposition, Jupiter’s moons and their respective shadows cross Jupiter pretty much in concert.
Bottom line: Late tonight, on the night of October 11-12, 2013, the first quarter moon sets in the west at about the time that Jupiter rises in the east. These two are opposite each other in the sky tonight.