If your pre-dawn sky is clear early Saturday (Sept. 28), you'll be treated to a lovely celestial scene as a fat, waning crescent moon will appear to pass close by the largest planet in our solar system: Jupiter.
The moon, which will be about 38-percent illuminated Saturday morning, will appear just 5 degrees below and to the right of Jupiter. Your clenched fist held at arm's length covers about 10 degrees of the night sky, so the moon and Jupiter will appear spaced roughly one-half fist apart.
Jupiter, or "Big Jupe" as I like to call it, will appear to the eye as a brilliant non-twinkling star. Astronomers measure the brightness of night-sky objects on a magnitude scale in which lower numbers denote brighter objects, with negative numbers given to exceptionally bright objects. [Spectacular Night Sky Photos for September 2013 (Gallery)]
On Saturday morning in clear conditions, Jupiter should shine with a magnitude -2.2, meaning that it will appear more than twice as bright as Sirius, the brightest star in the sky. Although they will not be particularly close together, both moon and Jupiter will make for an eye catching pair during the early hours of Saturday.
Where and when to see Jupiter
Jupiter currently rises around 12:20 a.m. local time, but will be rising two hours earlier by Halloween. About two hours after its appearance in the night sky, Jupiter will shining about 20 degrees above the horizon in the east-northeast direction. It should be the brightest star-like object in the sky.
Jupiter is currently in the middle of the constellation of Gemini near the 3.5-magnitude star Delta Geminorum (also known as Wasat) which marks the waist of the twin brother Pollux and is also situated above and to the left of Orion, the Hunter — winter constellations that will offer late-night Jupiter watchers an early fall preview of their cold splendors.
Interestingly, on the morning of Oct. 4, Jupiter will pass very close — just over a one-tenth of a degree — from Wasat, a sight worth looking for in binoculars or a small telescope.
See Jupiter through telescope
Jupiter shines regally high toward the south and highest during morning twilight this month, a most opportune time to try and study the details of its rich cloud features.
If you do turn a telescope to Jupiter, notice that the planet's eastern limb is brighter than its western side. Jupiter is still more than three months from opposition (which comes on Jan. 5), so the sun lights it at an oblique angle as seen from Earth. This phase effect, called "quadrature" will be greatest on Oct. 12, when Jupiter is exactly 90 degrees west of the sun and technically its disk appears only 99-percent lit. This geometry also lets us see the eclipses of its four big Galilean moons better.
A final note regarding the moon/Jupiter pairing on Saturday morning: Keep in mind that what you are seeing is an illusion of perspective. The moon is about 248,000 miles (399,000 km) away, while Jupiter is almost 1,980 times farther away at a distance of 491 million miles (790 million km).
Editor's note: If you snap an amazing photo of the moon and Jupiter, or any night sky view, that you'd like to share for a possible story or image gallery, please contact managing editor Tariq Malik at firstname.lastname@example.org.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmer's Almanac and other publications, and he is also an on-camera meteorologist for News 12 Westchester, N.Y.Follow us @Spacedotcom, Facebook and Google+. Original article on SPACE.com.
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