Astrology

Neptune reaches opposition on September 26, its brightest and closest approach of the year

Earth will pass directly between the sun and Neptune at 2 UTC on September 26, 2026, or 9 p.m. CDT on September 25, putting the eighth planet at its brightest and closest point to Earth for the year, EarthSky reports. The alignment, called opposition, means Neptune rises in the east as the sun sets in the west and stays visible all night.

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Neptune stays faint even at its best#

Despite the close approach, Neptune shines at only magnitude +7.8, too dim to see without optical aid, EarthSky and BBC Sky at Night Magazine both report. EarthSky puts Neptune at 240 light-minutes, or 28.9 astronomical units, from Earth on September 26, while Astronomy magazine had listed its brightness as magnitude 7.7 heading into opposition on September 25.

Through a telescope Neptune's disc spans just 2.4 arcseconds, and BBC Sky at Night Magazine says a 150mm telescope is needed to see it, with a scope around 200mm typically required to pick out its largest moon, Triton, though stable conditions can favor smaller apertures. EarthSky similarly notes Triton becomes visible in medium-size telescopes.

Viewing this year comes with a complication: a nearly full moon, more than 99 percent lit, will sit close to Neptune on the night of opposition, Astronomy.com reports. The outlet suggests using binoculars or, ideally, a telescope to look about 7 degrees east of the moon, or alternatively about 9 degrees west-southwest of Saturn, to find the planet.

Saturn and a nearby asteroid share the view#

Neptune sits in the constellation Pisces near Saturn, and BBC Sky at Night Magazine places it 9 degrees west-southwest of the ringed planet, a marker also confirmed by Astronomy.com. EarthSky notes that Neptune will remain in the evening sky through February 2027.

The same week brings a second opposition: asteroid 192 Nausikaa reaches its own opposition at 6 p.m. EDT on September 30 in Pisces, just north of where Saturn and Neptune appear, Astronomy.com reports.

What astronomers mean by opposition#

Opposition happens only for planets with orbits larger than Earth's, when the sun, Earth and the planet form roughly a straight line, according to Wikipedia. At that moment a planet appears in apparent retrograde motion, rises around sunset, culminates near midnight and sets around sunrise, appearing fuller and brighter than usual, Wikipedia's entry on the phenomenon states.

Neptune orbits the sun once every 164.8 years at a distance of 30.1 astronomical units, according to Wikipedia, making it the solar system's farthest known major planet. BBC Sky at Night Magazine notes that for a planet as distant as Neptune, opposition brings only a small, barely perceptible gain in brightness and size compared with a closer world like Mars.

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