This is the month of the "great plunge" so far as Venus is concerned. It starts May still close to its highest possible altitude in the evening sky and setting about 3.5 hours after sunset, but with each passing night it will drop a bit lower and sets about 6 minutes earlier than the previous night.
By the time the month comes to a close, you'll need to look low to the west-northwest horizon almost immediately after sunset to see Venus; by then it will be setting only about a half hour after sundown. Like two ships passing in the twilight, Mercury bolts up into view during the last 10 days of the month and engages Venus in a relatively close conjunction on the 21st, then continues to climb higher as the goddess of love sinks lower.
We then must wait until after midnight for the rising of the dynamic duo of Jupiter and Saturn and, between roughly midnight and dawn, an ever-brightening Mars.
In our schedule, remember that when measuring the angular separation between two celestial objects, your clenched fist held at arm's length measures roughly 10 degrees wide. Here, we present a schedule below which provides some of the best planet viewing times as well directing you as to where to look to see them.
On May 4, the innermost planet Mercury is on the far side of the sun from us, at superior conjunction. Six days later the scorched world reaches perihelion, its closest point in space to the sun. The nearer a planet is to the sun, the faster it moves in orbit, and the extra speed hurls Mercury up into view in our evening sky by about the 17th, seemingly hurrying on a course for a rendezvous with Venus on the 21st.
On May 24, concentrate low near the west-northwest horizon about 45 minutes after sunset. Venus will be there, albeit lower than it was just a few days ago. Mercury has now shifted 5.5 degrees to Venus's upper left. And 6.5 degrees to the upper left of Mercury you'll sight a slender waxing crescent moon.
The "evening star" is outstandingly conspicuous at the beginning of May, shining at its maximum brilliance of magnitude -4.7. (Magnitude is a logarithmic scale of brightness used by astronomers, with lower numbers denoting brighter objects and negative numbers indicating exceptionally bright objects.) This is due to Venus achieving on April 27 its "greatest illuminated extent" — the largest area of sunlit surface in square arcseconds.
On May 1, Venus displays a disk that's 24% illuminated. Viewers at latitude 40 degrees north can see it 35 degrees high at sunset and still 27 degrees up when twilight is deepening 45 minutes after sunset. What's more, Venus doesn't set until almost 3.5 hours after the sun, around 11:20 p.m. local daylight time. But the fall of Venus during the rest of the month is breathtaking. By May 15 the planet is about 24 degrees high at sundown and sets nearly 2.5 hours after the sun. By May 31, noticeably less bright, Venus is only about 4 degrees high at sundown and goes below the horizon only about 25 minutes after the sun.
What is happening in space is that Venus is rounding the bend on the near side of its orbit to us. This puts it ever closer to the sun-Earth line and causes the planet's outline to loom bigger, while the sunlit part facing our way grows ever thinner. Beginning May 17, telescopes (or even mounted binoculars) will show the disk of Venus less than 10% illuminated but only about 3% the apparent diameter of the moon. That would be the best time for those with exceptional eyesight to try the rare feat of glimpsing Venus's crescent with the naked eye.
Related: Examining the phases of Venus
To see the crescent and any subtle cloud features of Venus sharply in the telescope, be sure to observe as early as possible when Venus is still relatively high. On May 21, Mercury catches up to Venus. About 45 minutes after sunset, look low near the west-northwest horizon. Venus will stand out against the twilight sky, glaring at magnitude -4.2. Mercury will be 1.1 degrees below and slightly to the left. Although shining at a very bright magnitude of -0.6, that's only 3.6% the radiance of Venus. Binoculars will make sighting Mercury easy, although you should also be able to pick it out with your naked eye.
The Red Planet starts the month at magnitude +0.4; nearly as bright as Procyon, the eighth-brightest star in the sky. By month's end, however, it will glow at magnitude 0.0, surpassing the fifth-brightest star Vega in brightness.
Mars is also rising earlier: around 2:50 a.m. local daylight time on May 1 and an hour earlier by May 31. The best time to view it in telescopes is at morning twilight, when it has climbed respectively high in the southeast. Mars is still a little too small to show distant features in most telescopes. On May 31, its distance from Earth will be 94.3 million miles (151.7 million km). On the morning of the 15th, Mars is the very bright orange-yellow "star" hovering about 4 degrees to the lower left of the moon.
The gas giant Jupiter starts rising before midnight (local daylight time) by the last week of May. But the best time to see it in telescopes is when it's highest in the sky around the beginning of morning twilight. Jupiter halts its direct motion (eastward relative to the stars) on May 14 and starts drifting back toward the Teapot of Sagittarius.
The king of planets brightens from magnitude -2.3 to -2.6 over the course of this month. The highly regarded sky calculator Jean Meeus of Belgium invented the term "quasi-conjunction" as an approach of two bright planets within 5 degrees of each other, but without a conjunction in right ascension. All through May, Jupiter and Saturn fit this criteria, but on the morning of May 18 at 1 a.m. EDT (0500 GMT) is the moment of the least separation (4.7 degrees) between these two planets this month. Jupiter is the brighter of the two, outshining its ringed neighbor by two magnitudes.
Located in the constellation Capricornus, Saturn rises above the east-southeast horizon around 1:45 a.m. local daylight time at the beginning of May and around 11:40 p.m. by month's end. By the time dawn begins to brighten, Saturn has attained the fairly respectable altitude of 25 or 35 degrees depending on your latitude; the farther south you are the better.
If you can pull yourself out of bed at this hour, Saturn with its stunning ring system is especially worth examining in a telescope. On May 12, night owls who are out during the wee hours of this morning can look for a gathering between a waning gibbous moon, Jupiter and Saturn. The moon will be situated about 3 degrees below Jupiter and 6.5 degrees to the lower right of Saturn.
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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications in New York's lower Hudson Valley. Follow us on Twitter @Spacedotcom and on Facebook.