The night sky in July 2015

posted 2 Jul 2015, 05:13 by Pete Collins   [ updated 2 Jul 2015, 06:33 ]
by Anne Holt

Sunset:  July 1st - 21.41   July 31st - 21.07

Now that we're past the shortest night, or longest day as some people insist on calling it, we can start to look forward to more hours of darkness.   This will be very welcome as for the last few weeks the sky seems to have been almost always light or cloudy - and often both.

In July there will be two full moons.  The second of these, on 31st, may or may not be a blue moon, depending on which definition you use.  It qualifies if you regard the second full moon in any calendar month as a blue moon but not if you use the older definition of the third full moon in any season where there are 4.  These, now sometimes called black moons, can only occur in February, May, August or November - the months before an equinox or solstice.

The Earth is at aphelion (the point in its orbit when it is furthest from the sun) on July 6th.

And, of course, New Horizons makes its closest approach to dwarf planet Pluto on 14th, after a jouirney just 5 days short of 9 and a half years.

Constellations


The Summer Triangle (made up of Vega in Lyra, Deneb in Cygnus and Altair in Aquila) is now quite high in the southern half of the sky. Cygnus, with its Northern Cross asterism, and Lyra are particularly prominent.

At the start of the month Pegasus, followed by Andromeda, is rising in the early hours.

As always during the summer months, it isn't the best time to see the zodiac constellations or planets as the ecliptic never gets very high in the sky.  However, if you do happen to visit a dark sky site over the next few months you should be rewarded with good views of the Milky Way high overhead running through Cygnus and down to Sagittarius just above the southern horizon.

Planets

Venus is at mag -4.4,  still a brilliant evening object at the start of July, when it is close to Jupiter in the twilight, setting around 11.30 -  before the western sky has darkened fully.  On 1st they are separated by only half a degree.
It is now sinking rapidly in the West and by month end will be invisible as it sets only ten minutes after the sun.

Jupiter, in Leo, is mag -1.8, visible in the twilight in early July but overshadowed by the much brighter Venus. 
On 18th, Jupiter, Venus and the very thin crescent moon will form a triangle low in the west, but will set while the sky is still quite light.  Binoculars may be needed to see Jupiter but all 3 objects should be in the same field of view of a low powered instrument.

Saturn, in Libra,  mag 0.4.  Visible in the south as the sky darkens but still very low - only 22 degrees above the horizon at its highest point.  The rings are still nicely tilted, as they will be for some time.

Neptune, in Aquarius, mag 7.8, is rising at about 11pm and will reach its highest point in the sky just before dawn.

Uranus, in Pisces, mag 5.8, rises about an hour after Neptune.  On 9th it will be just 2 degrees north of the not quite third quarter moon. 

Mercury starts July as a morning object, rising an hour before the sun and not easy to see, even at mag zero.  By mid July it will have brightened to -1.4. On 9th it will be only 9 arcminutes from the fainter, mag 1.6,  Mars.  However Mars will be almost impossible to see with the naked eye and, because it is such a short time before sunrise, it is too dangerous to attempt to use binoculars.   Superior conjunction is on 23rd, so Mercury will not be visible for the second half of the month.

Mars is barely visible in the dawn skies this month.

Dwarf planet Pluto is at opposition on 6th - just 8 days before it has its first close encounter with a visitor from Earth.
It is in Sagittarius, sitting in the bowl of the teaspoon asterism (just above the teapot asterism), and at mag 14.4, theoretically visible through large amateur telescopes.  However, the best way to find it will be to take images a few days apart and look for a faint object which has moved against the background stars - just as Clyde Tombaugh did in 1930.

Meteor showers

Not very good for meteors this month.  The 2 main showers both  have a very low radiant and are better seen from more southerly latitudes.  And, if that wasn't bad enough, they both peak around the time of the full (maybe blue) moon.

Delta Aquarids, active July 15th to August 25th, peak July 28/29th,  ZHR  15. Faint, medium paced meteors with no persisting trails.

Alpha Capricornids, active July 5th to August 15th, probable peak August 1st, though this shower does not have such an obvious peak as most others.   ZHR  5,  slow moving meteors, could include bright fireballs.

The best chance of spotting a few meteors might be from a few early Perseids, which are active between 13th July and 26th August, even though the peak is not until mid August.


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