The Hunter’s Moon for the Northern Hemisphere will come tomorrow night (October 18, 2013), but there will be a bright full moon in the sky tonight, too. Watch for it high in the east as soon as the sun goes down.
And, despite what we show you on the sky chart at the top of this post, don’t expect to see the planet Uranus in the glare of tonight’s almost-full Hunter’s Moon. Uranus, the seventh planet outward from the sun, appears – at best – as a faint speck of light to the eyes alone on a dark, moonless night. You definitely must have exceptional vision to see this distant world without an optical aid, even under the best conditions. Most people need binoculars and a good sky chart to see this world, which resides 20 times farther away from the sun than does our planet Earth.
Still, tonight’s moon can help you imagine the whereabouts of Uranus on the sky’s dome.
Uranus is a real oddity in that it goes around the sun “sideways,” with its rotational axis almost lining up with its orbital plane. In contrast, the rotational axis of our planet Earth is inclined about 23.5o out of perpendicular to our orbital plane.
The orbital planes of Uranus’ major moons pretty much coincide with the planet’s equatorial plane. That’s in spite of the fact that Uranus’ equatorial plane is nearly perpendicular to the plane of its orbit around the sun.
As a general rule, the major moons in our solar system orbit their parent planets over their respective planets’ equators. There are a few exceptions: Saturn’s moon Iapetus, Neptune’s moon Triton – and, perhaps most significantly to us earthlings: Earth’s moon.
Our moon doesn’t orbit the Earth above our planet’s equator (0o latitude). Rather the moon’s orbital plane is highly inclined to the Earth’s equatorial plane. This month, the moon’s orbital path takes the moon from an extreme of 19.5o south of the celestial equator on October 9 to a northerly extreme of 19.5o north of the celestial equator on October 23.
If the moon’s orbital plane – like that of Uranus’ moons – coincided with our planet’s equatorial plane, our moon would always rise due east and set due west – meaning no Hunter’s Moon in autumn.
However, the inclination of our moon’s orbital plane will cause the moon to rise further north along the eastern horizon each day for nearly all of the upcoming week. For the Northern Hemisphere, these more northerly moonrises bring about sooner-than-average moonrises, which is the legacy of the Hunter’s Moon.
Bottom line: The moon is one day from full on October 17. Watch for it in the east as soon as the sun goes down. The planet Uranus is near tonight’s moon on the sky’s dome, but don’t expect to see Uranus, which resides 20 times farther away from the sun than does our planet Earth.
Click here to find out more about tomorrow’s October 18 full Hunter’s Moon.