Sky ArchiveTonight

Waxing toward full Hunter’s Moon

Image above: Path of the 2016 Hunter’s Moon by Jacob Baker of Fall River, Massachusetts. A 16-shot composite, made of photos taken at 10-minute intervals over 2 1/2 hours.

Tonight – October 23, 2018 – as seen from the whole Earth, the moon is waxing, or showing us a bit more of its illuminated face each evening. If you’re in the Northern Hemisphere, it’s waxing toward a full Hunter’s Moon and the second full moon of autumn. If you’re in the Southern Hemisphere, it’s waxing toward the second full moon of springtime. For all of us, on October 23, the moon is near the planet Uranus. More about Uranus below.

The Hunter’s Moon is the full moon immediately after the Harvest Moon – the closest full moon to the autumn equinox. This year’s full Harvest Moon fell on the night of September 24-25, 2018, a touch more than two days after the September equinox.

For all of us, this month’s full moon will come on October 24 at 16:45 UTC; translate UTC to your time. In United States time zones, the moon reaches the crest of its full phase on October 24, 2018, at 12:45 p.m. EDT, 11:45 a.m. CDT, 10:45 a.m. MDT, 9:45 a.m. PDT, 8:45 a.m. AKDT (Alaskan Daylight Time) and 6:45 a.m. HST (Hawaiian Standard Time).

The times don’t really matter. No matter where you live worldwide, look for a full-looking moon in the east as the sun goes down over the next several evenings. This full or full-looking moon will cross our skies throughout the night, as seen from around the globe.

For us in the Northern Hemisphere, this Hunter’s Moon will be displaying its unique features, characteristic of this time of year. That is, the inclination of our moon’s orbital plane to the celestial equator will cause the moon to rise further north along the eastern horizon each day for nearly all of the upcoming week. For northerly latitudes in the Northern Hemisphere, these more northerly moonrises reduce the lag time between successive moonrises, which is the legacy of the Hunter’s Moon.

Meanwhile, in the Southern Hemisphere, the opposite is taking place. The more northerly moonrises along the horizon mean a longer-than-average time between successive moonrises, from night to night, over the coming nights.

On October 23, 2018, the moon shines brightly near Uranus on the sky’s dome. Uranus can be seen with the eye alone, theoretically, but it’ll be tough in the glare of such a bright moon.

The October 23, 2018, moon is near Uranus. The chart above shows their relative locations, as seen from North America. We’ll see Uranus to the east (left) of tonight’s moon; we’ll see them arcing across the sky. From other locations, their orientation with respect to each other and the horizon will be different, but – for the whole globe – the moon is near Uranus on this night.

One caution … don’t expect to see Uranus with the eye. This faint world will be lost in the glare of the almost-full moon.

Even on a dark moonless night, Uranus appears – at best – as a faint speck of light to the eye alone. You need exceptional vision to see this distant world without an optical aid, even under the best conditions. Here’s a good sky chart, if you want to see Uranus.

Just be aware Uranus is up there, near this nearly full moon. And think about the fact that 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 23.5 degrees 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 above 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 (0 degrees latitude). Rather the moon’s orbital plane is inclined to the Earth’s equatorial plane. The moon’s orbital path took the moon from its maximum declination of 20.9 degrees south of the celestial equator on October 15, and then the moon will swing to its maximum declination of 21.3 degrees north of the celestial equator on October 30.

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. The Hunter’s Moon will come on October 24, as the moon is going eastward as well as northward in its orbit.

Uranus and moons
Near-infrared image of the ice giant Uranus , its rings and some of its moons. Image via European Southern Observatory.

Bottom line: To the eye, the moon will appear nearly full as the sun sets on October 23. Watch for it in the east as soon as the sun goes down. In fact, it has a bit more to go, and full moon is October 24.

October 23, 2018
Sky Archive

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