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The Sky This Week: Pluto reaches opposition

Catch the famous dwarf planet, the shadow of Earth, and a Full Moon in the sky from July 16 to 23.
RELATED TOPICS: OBSERVING
Summer evening sky with Earth's shadow and Belt of Venus
Wooden Ore Cart & Belt of Venus
Earth’s shadow sits below the glowing pink Belt of Venus in this summertime snapshot from July 2018.
Jeff Sullivan (Flickr)

Friday, July 16
By the time the sky grows dark after sunset, the constellation Lyra is already high in the east. The Harp is home to the famous Double-Double: A naked-eye double star in which each bright sun has a second companion.

Start at Vega, Lyra’s unmistakable magnitude 0 alpha star. From there, swing your gaze 1.7° east-northeast, where you’ll find a close pair of magnitude 4.6 stars. They’re separated by 3.5' (or 208"), which is just about the limit of naked-eye resolution for most people. Can you see the two distinct stars? The luminary to the north is Epsilon11) Lyrae, while the other is Epsilon2 Lyrae. Even if you can’t separate them by eye, virtually any pair of binoculars or a small telescope will split them.

Once you zoom in on the pair, bump up the magnification to at least 80x and you’ll see that both Epsilon1 and Epsilon2 have a companion. This is why they’re called the Double-Double. The components of Epislon1 are separated by 2.8"; Epsilon2 is separated by 2.2". Although the entire system is gravitationally bound for now, ultimately the pairs of Epsilon1 and Epsilon2 may go their separate ways.

Sunrise: 5:45 A.M.
Sunset: 8:27 P.M.
Moonrise: 12:32 P.M.
Moonset: 12:03 A.M.
Moon Phase: Waxing crescent (42%)
*Times for sunrise, sunset, moonrise, and moonset are given in local time from 40° N 90° W. The Moon’s illumination is given at 12 P.M. local time from the same location.
Finder chart for Pluto during July 2021
Pluto’s motion during July
Pluto moves slowly westward this month through a region of relatively dim stars. This chart shows stars down to magnitude 15.
Astronomy: Roen Kelly

Saturday, July 17
First Quarter Moon occurs at 6:11 A.M. EDT.

The dwarf planet Pluto reaches opposition at 7 P.M. EDT tonight. You’ll find it in the south, rising around 8 P.M. local time amid the stars of Sagittarius the Archer. Give it an hour or two to climb out of the turbulent air near the ground a bit, as well as for the sky to grow dark enough to search out the tiny, magnitude 14.3 world. You’ll need at least an 8-inch telescope to spot Pluto as it slowly moves westward against the background stars. Checking back every hour or two should reveal its subtle motion. There are no bright stars in the vicinity; you can use the star chart above to find Pluto anytime this month.

Unfortunately, the bright First Quarter Moon tonight will make tracking Pluto a bit more challenging. Try to find an observing site with minimal to no light pollution to improve your chances, or wait to search until later this month or early next, when the Moon is no longer so bright.

Nearby, asteroid 6 Hebe also reaches opposition at 7 P.M. EDT, located in Aquila the Eagle. You should have less trouble finding the brighter magnitude 8.5 world near the border of Aquila and Sagittarius, about 3.5° south of 5th-magnitude Kappa (κ) Aquilae.

Sunrise: 5:46 A.M.
Sunset: 8:26 P.M.
Moonrise: 1:43 P.M.
Moonset: 12:29 A.M.
Moon Phase: Waxing gibbous (53%)

Sunday, July 18
Asteroid 2 Pallas is stationary in the constellation Pisces at 4 P.M. EDT today. The second asteroid ever discovered, it is the third-largest asteroid in the Main Belt and orbits the Sun once every 4.6 years.

Tonight, Pallas sits north of the Circlet in Pisces, just at the border between Pisces as Pegasus. The magnitude 10 world will need binoculars or, depending on your sky conditions, a small telescope to easily find. It’s located about 2.3° east-northeast of 4th-magnitude Theta (θ) Piscium, and is roughly even with the star above the horizon as they rise late tonight around 10 P.M. local time. You’ll want to let them climb higher for the best views, which will occur after midnight and into the early hours of tomorrow morning.

Sunrise: 5:47 A.M.
Sunset: 8:25 P.M.
Moonrise: 2:55 P.M.
Moonset: 12:57 A.M.
Moon Phase: Waxing gibbous (65%)

Monday, July 19
At 12 A.M. EDT this morning, just as the date changes from July 18 to 19 on the East Coast of the U.S., Jupiter’s moon Io is poised to transit the planet’s large disk. The dark blot of the tiny moon’s shadow is already one-third of the way across the planet, and will take another hour and a half to reach the western edge. Meanwhile, Io itself takes about 2 hours and 20 minutes to cross the face of Jupiter, which stretches 47" across. Even a relatively small scope should show the sequence beautifully, thanks to Jupiter’s large size and bright magnitude –2.8 glow.

Jupiter’s Great Red Spot, an ongoing storm roughly the size of Earth, is also visible on the planet’s disk for much of the transit, slightly ahead (west) of Io’s shadow. Look for other cloud features and how they change as the planet turns — its fast rotation rate of just under 10 hours means motion is visible over just a short period of time.

All three remaining Galilean moons sit west of Jupiter: from west to east are Europa, Ganymede, and Callisto.

Sunrise: 5:47 A.M.
Sunset: 8:25 P.M.
Moonrise: 4:10 P.M.
Moonset: 1:30 A.M.
Moon Phase: Waxing gibbous (75%)

Tuesday, July 20
Mercury has been growing brighter over the past two weeks; but even so, its visibility is waning as its elongation shrinks. If you’re up before the Sun, you may spot the small planet shining brightly at magnitude –1 about 45 minutes before sunrise, when it’s just 3° high and rising between the two figures of Gemini the Twins. About 4.5° above Mercury shines Mebsuta, Gemini’s 3rd-magnitude epsilon star. 13° to this star’s northeast (lower left on the sky) is Castor, one of the Twins’ heads. Pollux will clear the horizon about 10 minutes later. The Sun is hot on both stars’ heels.

This is one of your last chances to catch Mercury in July; over the next week, it will continue to sink toward the Sun, reaching superior conjunction next month.

Sunrise: 5:48 A.M.
Sunset: 8:24 P.M.
Moonrise: 5:25 P.M.
Moonset: 2:08 A.M.
Moon Phase: Waxing gibbous (85%)

Wednesday, July 21
The Moon reaches perigee at 6:24 A.M. EDT, when it will sit 226,503 miles (364,521 kilometers) from Earth. Perigee is the closest point to Earth in the Moon’s not-quite-circular orbit; the farthest point is called apogee.

Venus passes 1.2° north of Regulus, Leo the Lion’s alpha star, at 3 P.M. EDT. By half an hour after sunset, both are visible in the western sky. The planet — blazing a bright magnitude –3.9 — is now 1.1° northeast of the 1st-magnitude star, far outshining it. Meanwhile, Mars — a much dimmer magnitude 1.8 — sits farther west along the ecliptic, nearly 5° from Venus and closer to the horizon. Binoculars or a small scope will show the Red Planet’s 4"-wide disk, while Venus shows off a 12"-wide, 85-percent-lit crescent.

Sunrise: 5:49 A.M.
Sunset: 8:23 P.M.
Moonrise: 6:37 P.M.
Moonset: 2:54 A.M.
Moon Phase: Waxing gibbous (92%)
Map of the Moon's north lunar region during July 2021's northern libration
Lunar north pole
Thanks to libration, several craters we rarely see are on display.
NASA’s Scientific Visualization Studio

Thursday, July 22
From our Earthbound perspective, the Moon appears to bob up and down across the ecliptic (the plane of the solar system) as it orbits our planet. That means sometimes, we get a better view of its northern or southern polar regions, depending on our perspective. Today, the Moon hits its lowest point — called a northern libration — giving us more of a top-down view than usual. The angle reveals a swath of terrain near the pole, including craters such as Rozhdestvensky, which sits at 87° latitude on the farside. Other craters on view include Nansen, Peary, and Byrd.

A small telescope is enough to net you this bird’s-eye view; higher magnification will cut down on the amount of light you’re getting, which will help to dim our satellite’s bright glow. If you aren’t able to observe tonight, don’t worry — you’ll get another chance next month, when the bobbing motion once again coincides with the Full Moon.

Sunrise: 5:50 A.M.
Sunset: 8:23 P.M.
Moonrise: 7:42 P.M.
Moonset: 3:51 A.M.
Moon Phase: Waxing gibbous (97%)

Friday, July 23
Full Moon occurs at 10:37 P.M. EDT. Our satellite rises about two hours earlier, shortly after the Sun sinks below the horizon. That means you’ll see the Moon rising amid Earth’s shadow, which also rises as the Sun moves behind the curvature of our planet.

The shadow is visible as a dark band of sky near the horizon and typically sits beneath a brighter band or arch of pinkish-orange sky, known as the Belt of Venus, which separates it from the still-blue sky above. This pink glow is caused by scattered sunlight, as particles in the atmosphere preferentially scatter longer (redder) wavelengths — the same reason the sunrise or sunset is red.

You can see both Earth’s shadow and the Belt of Venus either after sunset or before sunrise, opposite the location of the setting or rising Sun in the sky.

Sunrise: 5:51 A.M.
Sunset: 8:22 P.M.
Moonrise: 8:36 P.M.
Moonset: 4:56 A.M.
Moon Phase: Full
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