The Night Sky This Month: May 2017

Spring Constellations
Image Credit: Duncan Moran

In May, take time to venture out in the early evening to see such magnificent sites as the lionesque constellation of Leo making its way from east to west across the night sky, followed by Virgo, the 2nd largest constellation, which still holds court with the King of planets, Jupiter. These two constellations brightest stars are Regulus and Spica respectively, which together with Arcturus in Bootes higher up in the northern sky form a familiar star pattern called the Spring Triangle, although Denebola in Leo is also often used as one of the triangle’s points. By the end of the month, however, it will be another huge asterism that will ascend in the east before dominating overhead during the summer months, namely the Summer Triangle

Meteor Showers

The Eta Aquarid meteor shower associated with debris from Halley’s Comet is among the most spectacular meteor showers when viewed from the southern hemisphere, when up to 30 meteors per hour can be seen. Despite this, in the northern hemisphere it might be possible to view some meteors emanating from the constellation of Aquarius in the pre-dawn hours on the 6th of the month, when the meteor shower is expected to peak. However, the Moon will be nearly full at this time, which will interfere with the few meteors that may be seen in the early hours.

Moon Phases

First Quarter: 3rd
Full Moon: 10th
Last Quarter: 19th
New Moon: 25th


– Mercury is not observable during May, due to its position close to the Sun.

– Venus starts the month rising just before dawn in the east, but gains altitude as the month wears on. On the first day of the month, the planet will show a crescent about 38 seconds of arc high, and will be about 27% illuminated, giving it a magnitude of -4.7, which is about as bright as it ever gets. Towards the end of May, the planet’s brightness will increase somewhat to magnitude -4.5, its angular diameter will reduce to 25 seconds of arc, and its illumination will increase to 48%. By month’s end, Venus will also be close to its maximum elongation from the Sun, at 46 degrees, which position it will reach on the third day of June. However, due to the shallow angle between the horizon and the ecliptic at this time of year, Venus will not rise above about 16 degrees at dawn.

– Mars starts the month in the constellation Taurus, forming a shallow triangle with the star Aldebaran towards its lower left, and the beautiful Pleiades cluster to its lower right. During the first days of the month, Mars has an elevation of around 11 degrees above the western horizon at sunset; however, this elevation decreases to only about 5 degrees by the end of the month, at which time Mars becomes lost in the Sun’s glare. As a result of its passing behind the Sun, Mars will not be visible for the entire summer.

– Jupiter reached opposition on April 7th, which means that the King of the planets now rises at dusk, and transits the meridian at an elevation of around 36 degrees in the late evening. Moving “backwards” through the constellation Virgo, Jupiter starts the month about 9 degrees or so from the star Spica, the brightest star in Virgo. Jupiter’s disc will shrink somewhat from 43.5 to 40.8 seconds of arc as the month wears on, while its brightness will dim from magnitude to -2.4 to magnitude -2.3.

Note that if seeing conditions allow on the night of the 15th, all four Gallilean Moons will be visible lined up on one side of the planet. The month of May also offers many opportunities to observe the Great Red Spot, which interestingly, has been shrinking quite dramatically over the last century or so. At the start of the last century, the GRS spanned a full 40,000 km, into which the Earth could fit three times. Recent measurement however, show the GRS to be only about 16,500 km across, but worse, the rate of shrinkage appears to be accelerating from the roughly 580 km/year it was a few years ago.

– Saturn is now in the western reaches of the constellation Sagittarius, where it rises at about 11:30pm BST to reach its highest elevation before dawn at about 4 Am BST. As the month wears on however, the planet will rise progressively earlier from around 9:30 PM BST, to transit the meridian at about 2 PM BST by the end of the month. Saturn’s disc will grow slightly from 17.8 to 18.3 seconds of arc during the month, while its brightness will increase somewhat from magnitude +0.3 to +0.1. Bear in mind that even though the ring system is inclined towards our line of sight by a full 26 degrees, the planet never rises above 18 degrees above the horizon for the rest of 2017, which means that it might become necessary to use a dispersion corrector to obtain clear views of the ring system.

Deep Sky Objects

– Comet Johnson makes an appearance during the last ten days of May, and since there will be no moonlight to spoil the view, a small telescope or even binoculars will easily reveal the comet as it crosses the constellation Bootes, on what would appear to be a collision course with the star Arcturus. With some luck, the comet might brighten to magnitude +6 or so, which if it does, will make it a hard object to miss with binoculars. The chart below shows the comet’s track through the constellation.

Messiers 35– Messiers 35, a big open cluster in the constellation Gemini, is an easy target for small telescopes, and even binoculars under dark skies. Look for this gem in the sky with a wide-field eyepiece at low power for the best views. Instruments of 8-inch aperture and lager will reveal a second, slightly smaller cluster (NGC 2158) nearby. Note however that this second cluster is four times further away than M35, and at least ten times older, which means that the two clusters are not physically related in any way.

– NGC 2903, a beautiful magnitude 9 spiral galaxy, is somewhat surprisingly not listed in either the Messier or Caldwell catalogues. It is however listed in the NGC with the number 2903, and it can be found with a medium-sized telescope in the constellation Leo, almost directly below the star lambda Leonis.

… Finally, as the nights continue to get later and shorter, the month of May will provide a good opportunity to view some true astronomical darkness before next June’s Summer Solstice when from the Northern Hemisphere sunset won’t occur until just a few hours before midnight.

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