The Harvest Moon lights the night, while distant Neptune reaches opposition.
With Venus, Mercury and Mars clustered too near the Sun to be visible, only two naked-eye planets grace the night sky: Jupiter and Saturn. Each world is a tremendously rewarding telescopic sight, but neither is currently at its best. Well past its June opposition, Jupiter sits roughly 20 degrees above the south-southwest horizon one hour after sunset. Saturn, on the other hand, culminates just before 9 p.m., local daylight time, with an altitude of roughly 23 degrees. Both altitude figures are for observers at the latitude of Toronto, Ontario. If you’re farther north, the planets will appear closer to the horizon. A low altitude doesn’t completely rule out good telescopic images, but it does mean there’s a substantial amount of air between you and the two planets, which means steady high-power views will occur far less often. The best strategy is to keep your scope’s magnification (and your expectations) reasonably low.
Neptune is at opposition today. That means the distant planet is at its brightest and up all night. (For more, see Weekend Stargazer, below.)
The Harvest Moon rises this evening at sunset. However, full Moon doesn’t officially occur until after midnight (12:33 a.m., EDT, September 14). Tonight’s full Moon also happens to be the smallest of 2019. Will someone dub this the “mini Harvest Moon?”
Evenings over the September 13 – 15 weekend are flooded with moonlight, but don’t let that stop you from hunting down Neptune. The most distant planet (unless you still count “dwarf planet” Pluto) is currently at its brightest, though that stat doesn’t change much throughout the year. Right now, Neptune is at magnitude 7.8, and when it’s less favourably positioned, it dips an imperceptible 0.2 magnitude, to 8.0. Either way, the planet is bright enough to be viewed in small telescopes—even when a full Moon is present.
Neptune transits the meridian (an imaginary line that joins north and south, and passes directly overhead) around 1 a.m., local daylight time. But you can start your search much earlier than that. The planet is making its way through the big, dim constellation Aquarius. Luckily, locating Neptune this weekend is not too difficult since it’s found just ¼ degree west-southwest of 4.2-magnitude Phi (φ) Aquarii. Centre Phi in your scope, crank up the magnification, and Neptune should be obvious as the brightest object near the star.
In a telescope, Neptune is merely a blue-green point of light. Just 2.3 arc seconds in diameter, it appears only slightly bigger than Jupiter’s largest moon, Ganymede, which is 1.6 arc seconds wide. You’ll have to use high magnification (200× or more) to resolve that pale pinpoint into a tiny disc. Detect the disc and you’ve confirmed your catch!
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