Ice giants on display in early autumn
Neptune and Uranus reach their 2013 peaks within six weeks of each other — on August 26 and October 3, respectively.
As summer ends and the nights turn longer and colder, observers should head out and look for the solar system’s two outermost major planets. Each planet reaches opposition (when the world lies on the opposite side of Earth from the Sun) within several weeks of each other this year. Uranus lies at this point in its orbit October 3, less than six weeks after its more distant sister planet, Neptune, which arrives at opposition August 26. Both will be easy to find and fascinating to observe these fall evenings, as opposition means a planet is at its closest to Earth for the year and therefore appears brightest in the night sky.
Neptune’s opposition only marks the middle of the planet’s prime observing season., however. In fact, the most distant planet lies so far from Earth (some 2.7 billion miles [4.3 billion kilometers]) that its appearance hardly changes during the next several months. It remains at its peak brightness — magnitude 7.8 — until mid-October and doesn’t fade below magnitude 7.9 until 2014. It also stands higher in the evening sky during autumn than in late summer.
You’ll need binoculars or a telescope to spot Neptune. It resides among the background stars of Aquarius, roughly midway between Sigma (σ) Aquarii (magnitude 4.8) and 38 Aqr (magnitude 5.4) throughout the summer and fall. These two stars lie 5° apart and will appear in the same binocular field. Turn a telescope on Neptune, and boost the magnification to see its tiny blue-gray disk.
Uranus is easier to locate. It spends 2013 not far from Neptune, wandering in a sparse region near the border between Pisces and Cetus. The planet shines at magnitude 5.7 at opposition October 3 (and is a scant 0.1 magnitude dimmer in August), bright enough to show up to naked eyes from a dark site. It’s much easier to view through binoculars, though. Uranus lies in the same binocular field as the magnitude 4.4 star Delta (δ) Piscium. At opposition, the planet lies 5° south-southwest of the star. On October 14, Uranus moves within 0.1° of a magnitude 6.4 sun. Crank up the magnification, and you’ll see the planet’s small blue-green disk.
Senior Editor Richard Talcott suggests trying to spot both ice giants in one night. “The planets’ proximity to each other provides a perfect opportunity to track them both down on any clear night in the next three months,” he says. “You won’t see any detail on their disks, but it’s still exciting to observe the only two major planets found since antiquity.”
Neptune Fast Facts
- Neptune is the solar system’s most distant planet, lying about 30 times farther away from the Sun than Earth, on average.
- Neptune is the most recently discovered planet, first spotted September 23, 1846.
- Neptune is the first planet discovered by prediction: French mathematician Urbain Le Verrier calculated its expected location, and German astronomer Johann Gottfried Galle observed it less than 1° away from that point.
- It takes Neptune 164.79 Earth years to orbit the Sun, but only 16.11 hours to rotate completely.
- Neptune’s color is a result of the abundant methane in its atmosphere.
- Neptune has 14 known moons. Astronomers discovered the latest one just this year from archival Hubble Space Telescope photos.
Uranus Fast Facts
- Uranus was the first planet discovered with telescopic aid, first spotted by British astronomer William Herschel on March 13, 1781.
- Uranus’ blue-green color is a result of the planet’s methane, which absorbs red wavelengths and reflects blue.
- Uranus completes one orbit in 84.01 Earth years, and it rotates once every 17.24 hours.
- Uranus has a ring system turns edge-on to Earth every 42 years due to the distant planet’s changing seasons.
- Planetary scientists have discovered 27 satellites orbiting Uranus and have named them all after characters in works by William Shakespeare and Alexander Pope.
- Scientists think a huge collision early in Uranus’ history knocked the planet on its side; its rotation axis is now nearly parallel to its orbit.
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