Here is a listing of celestial events you might want to look for during these spring nights in May. All times are listed as Eastern Daylight Time for the Northern Hemisphere.
Take note that we sometimes will use angular degrees to define the separation between two objects, such as (for example) the Moon and a bright planet. Keep in mind that the width of your clenched fist, held at arm’s length, measures roughly 10°.
May 1 – Venus is outstandingly conspicuous at the beginning of May, shining at its maximum brilliance. This is due to Venus achieving its “greatest illuminated extent” – the largest area of sunlit surface in square arc seconds on April 27th. On May 1st Venus displays a disk that’s 24% illuminated.
Viewers at latitude 40° north can see it 35° high at sunset and it’s still 27° up when twilight is deepening 45 minutes after sunset. What’s more, Venus doesn’t set until almost 3½ hours after the Sun, around 11:20 p.m. But the fall of Venus during the rest of the month is breathtaking. By May 15th the planet is about 24° high at sundown and sets nearly 2½ hours after the Sun.
By May 31st, a noticeably less bright Venus is only about 4° 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 17th, telescopes (or even mounted binoculars) will show the disk of Venus less than 10 percent illuminated but nearly 1/30 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. 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.
May the Fourth –Mercury is on the far side of the Sun from us, at superior conjunction. A superior conjunction occurs when a Solar System body, such as a planet (in this case, Mercury), asteroid, or comet, lies in a straight line with the Earth and the Sun, but is on the opposite side of the Sun from the Earth. See the diagram below. Six days later, it 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.
May 5 -This year’s Eta Aquarid meteor shower has the misfortune of reaching its maximum just two days before the full Moon, meaning all but the brightest of its meteors will likely be squelched by bright moonlight.
May 7 – May’s Full Flower Moon at 6:45 a.m. In this phase, the visible Moon is fully illuminated by direct sunlight. Though the Moon is only technically in this phase for a few seconds, it is considered “full” for the entire day of the event and appears full for three days. See how May’s Moon got its many names in our short video, below.
May 11-14 – If you’re out during the wee hours of the morning, look southeast in the predawn sky for a gathering between a waning gibbous Moon, Jupiter, and Saturn. (Mars appears off to their left and on the 14th and 15th, look for it closest to the half Moon.)
Jupiter starts rising before midnight 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 14th and starts drifting back toward the Teapot of Sagittarius. The king of planets brightens over the course of this month.
Saturn, in Capricornus, 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° 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.
May 14 –Last Quarter Moon at 10:03 a.m. In this phase, the Moon appears as a half Moon due to the direct sunlight, the illuminated part is decreasing toward the new Moon phase.
May 15 – Look for Mars, the very bright orange-yellow “star,” hovering about 4° to the lower left of a half Moon in the predawn sky. Mars is now brightening impressively and becoming prominent. It starts the month nearly as bright as Procyon, the eighth brightest star in the sky. By month’s end, however, it will glow will surpass the fifth brightest star, Vega in brightness.
Mars is also rising earlier: around 2:50 a.m. local daylight time on the 1st; an hour earlier by the 31st. 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 31st, its distance from Earth will be 94.3 million miles.
May 18 – 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 these criteria, but this morning at 1 a.m. EDT, is the moment of the least separation (4.7°) between these two planets this month. Jupiter is the brighter of the two, outshining its ringed neighbor by two magnitudes.
May 21 – New Moon at 1:39 p.m. In this phase, the Moon is not illuminated by direct sunlight and is completely invisible to the naked eye.
Also this evening, Mercury catches up to Venus. About 45 minutes after sunset, look low near the west-northwest horizon. Glaring Venus will stand out against the twilight sky. Mercury will be 1.1° below and slightly to the left.
Although shining very bright in its own right, Mercury glows with only 1/28 the radiance of Venus. Binoculars will make sighting Mercury easy, although you should also be able to pick it out with your naked eye.
May 24 – Once again, 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 5½° to Venus’s upper left. And 6½° to the upper left of Mercury you’ll sight a slender waxing crescent Moon.
May 29 – First Quarter Moon at 11:30 p.m. n this phase, the Moon looks like a half-Moon in the sky. One-half of the Moon is illuminated by direct sunlight while the illuminated part is increasing, on its way to becoming a full Moon.
Our schedule is adapted from “Skylog,” a regular feature appearing in Natural History magazine, written by Mr. Rao since 1995.