The Night Sky This Month: July 2017

Comet V2 Johnson
Photo of Comet V2 Johnson by José J. Chambó

The first real dark skies for the northern hemisphere occur only after about the 11th of July, which offers a perfect opportunity to view the Summer Triangle, an asterism made up of the bright stars Vega, Altair and Deneb, in the constellations of Lyra, Aquila and Cygnus respectively. Interestingly, a search for exoplanets in the area between Deneb and Vega has turned up 4,043 possible planets, of which about 50 or so are Earth-sized, and orbit their stars within their habitable zones. The full list of possible and confirmed planets in this area is available here.

Moon

In July, the full Moon is known as the Full Buck Moon in America, because it is at this time of the year that deer bucks’ antlers enter a high growth phase. Below are the Moon phases for July 2017:

First Quarter: 1st
Full Moon: 9th
Last Quarter: 16th
New Moon: 23rd

Planets

– Mercury sets soon after sunset for most of July. However, the little planet reaches its greatest eastern elongation from the Sun on the 30th of the month, when it will be 27.2 degrees away from the Sun. Look for Mercury low above the western horizon just after sunset during the last days of July.

– Venus dominates the pre-dawn sky during July, shining at magnitude -4.0 for almost the entire month. Rising at about 02:20 (BST) in the northeast, the planet reaches an elevation of 19 degrees above the eastern horizon on the 1st, but steadily gains height as the month wears on to reach an altitude of 25 degrees or so by month’s end. Look for Venus about 8 degrees south-eastward of the Pleiades during the first days of the July. As the month progresses, Venus will move between the Pleiades and the star Aldebaran to pass about 3 degrees to the upper leftward of Aldebaran on the 14th. Although Venus’ angular diameter will reduce from 18 arcseconds to 15 arcseconds, its illumination will increase from 63% to 74%, which means its brightness will remain largely unchanged during July.

– Mars is not visible at this time, being washed out by the Sun’s glare, and it will only reappear as a morning object at the end of August/beginning of September.

– Jupiter remains a conspicuous magnitude -2.0 object low above the south-western horizon before sunset as it continues its eastward motion through the constellation Virgo. Look for the King of the Planets close to the Moon on the first day of July, and again on the 28th, by which time it will be even lower above the west-south-western horizon when it sets at about 23:01 (BST) on the last days of July.

– Saturn starts July rising at 19:48 (BST) but rises progressively earlier as the month wears on, to rise at 17:43 (BST) on the 31st. The planet reached its point of closest approach to Earth on June 15th, which means that relatively speaking, it is still fairly close to us as it transits the constellation Sagittarius in which it now occupies an apparent spot just above the centre of the Milky Way. However, although it remains low on the horizon, it is nevertheless the brightest object low above the southern horizon, even though the planet will dim slightly from magnitude 0.1 to 0.3 as the July progresses.

Meteor Showers

July sees the arrival of the Southern Delta Aquarid Meteor Shower, which is suspected to be debris from Comet 96P Machholz. Although this meteor shower favors observers in the southern hemisphere, observers in northern tropical latitudes can observe the poorly-defined peak best between July 27th, and July 30th. Radiating from the star Skat in the constellation Aquarius, this shower usually delivers a maximum of around 15 -25 or so meteors per hour in the small hours just before dawn.

Deep Sky Objects

July brings the parabolic Comet V2 Johnson to within sixth or seventh magnitude, meaning that from a dark site, it will be visible in binoculars and even to the naked eye if seeing conditions are exceptionally good. Look for the comet in the constellation Virgo, where it will be at magnitude 7.5 only about 0.5 degrees to the northward of the star Kappa Virginis during the first days of July. The first really good opportunity to view the comet will occur on July 11th, when it will be close to the Hydra/Libra border, but still in Virgo, from whence it moves into Hydra on the 17th of the month. By the end of the month, the comet will be located less than one degree from the face-on spiral galaxy NGC 5556. By the end of July, Comet Johnson will become visible only to southern hemisphere observers as it continues its southward motion.

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