Watch Venus and Uranus Double Up Beneath a Comet

After an occultation of Rho Leonis by the Moon, watch Venus and Uranus pair up in a weekend conjunction just 10° from Comet PanSTARRS (C/2015 ER61). 

The alarm went off at 3:30 a.m. Checking to see if the sky was clear, I thought I saw thick fog illuminated by a neighbor's yard lights and nearly returned to bed. But after rubbing my eyes and looking a second time, I realized the glow was dawn and the sky cloudless. Will I ever get used to how crazy early summer twilight begins?

Dawn brings many good things this season including the reassuring sight of Venus every single clear morning. It currently rises in eastern Pisces around 3:45 a.m. local time and stands some 13° high an hour before sunrise from the central United States. Through a small telescope the planet appears 48% illuminated but will expand to a neat 50% on June 3, when it reaches greatest western elongation from the Sun.

By happy coincidence, that's also the day Venus will be in close conjunction with its distant sibling Uranus. Neither planet is in a particular hurry, so they linger together for several mornings, coming closest on the 3rd, when they're just 1° 42′ apart. If you've never seen a conjunction between these two, it makes a wonderful study in opposites. With a surface temperature of 864°F (462°C), Venus is the consistently hottest planet in the Solar System. Uranus, on the other hand, is the coldest.

What? Colder than more distant Neptune? Yes, because Uranus's core gives off less heat than the cores of the other giant planets, so its cloud tops register a colder temperature. Try –357°F (–216°C), about four degrees chillier than Neptune. There's a certain poetry, then, in the June 3rd tête-à-tête of warmest and coldest, a pairing of planets so dissimilar they somehow complement one another. Opposites really do attract.

The diagram above shows the view 45 minutes before sunrise, but given that Uranus will magnitude +5.9, it wouldn't hurt to start earlier, say an hour and a quarter before sunrise, to take advantage of a darker sky. This is especially true if you live in the northern U.S., where dawn begins early! A pair of 35-mm or 50-mm binoculars magnifying 7–10× should be all you need to pick out pinpoint Uranus slightly above Venus in the same field of view.

You'll see a brighter 4th-magnitude star just to left of Venus with the delightful name of Tocularis Septentrionalis (a.k.a. Omicron Piscium). It roughly translates to "Northern Thread," an old Arabic reference to the flaxen threads or cord tying the fish together at either end of the constellation. The name rat-a-tats off the tongue like one of my other favorite stars, Zubenelgenubi.

If bad weather interferes on the day of conjunction, don't worry: Venus and Uranus will remain within 2.5° of each other from June 1–4. Use the panel maps below to help you know just where to look. The maps also plot the position of Comet PanSTARRS (C/2015 ER61), which has been hanging around Venus low in the eastern pre-dawn sky for some time now. Extinction from low altitude may preclude you seeing the 8th-magnitude object in binoculars, but a 6-inch scope should show it easily.

For the best views of the comet, start your morning observing session 90 minutes to 2 hours before sunrise, when the sky is still dark.

That's not all. Die-hards can try digging up Mercury from the horizon haze starting about 45 minutes before sunrise. Look for it with binoculars two and half fists (26°) to the lower left of Venus. At magnitude +0.4, it's relatively bright, but it stands only about 3–4° high.

Rho, Rho, Rho Your Scope

Several nights before the conjunction — tonight (May 31st) as a matter of fact — the dark limb of the first quarter Moon will occult the 3.8-magnitude star Rho (ρ) Leonis for much of the United States, Canada, and Mexico. This should be an easy and fun event to see in a small scope, since the relatively bright star will encounter the Moon's dark limb, making for a strong contrast of dark and light. The reappearance on the bright limb approximately an hour later will be more challenging.

Rho Leonis may look ordinary, but it's one of the "hottest, bluest, most massive stars you can see with the naked eye," according to astronomer James Kaler. The supergiant is roughly 3,650 light-years away with a luminosity 165,000 times that of the Sun and a mass 23 times solar. Rho also keeps a close companion on such a tight 0.01″-long leash that even the largest telescopes can't pry the two apart. Might some variation in light be perceptible as the Moon covers one and then the other star in rapid succession?

From the slick trick of a puny Moon making a massive supergiant vanish to a planetary pairing under the eye of a comet, it's all manna from heaven for the ardent skywatcher.