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Mercury
For observers at northern temperate latitudes, September offers the best opportunity this year to see Mercury in the morning sky. During the second week of the month, it should be possible to spot the elusive planet low down above the eastern horizon just before sunrise. Mercury appears slightly higher in the sky each following morning until greatest elongation is reached on September 12th, when it will be 17.9 degrees west of the Sun. As it draws back towards the Sun the planet remains visible, until finally lost to the bright twilight towards the end of the month. Observers should note that Mercury increases in brightness from magnitude +0.5 to -1.3 during the visibility period.
In the same region of sky as Mercury are Venus, Mars and first magnitude star Regulus (α Leo - mag. +1.4). From about September 5th to 12th, Mercury forms a close grouping in the bright twilight with Regulus and Mars (mag. +1.8). At times, binoculars will be required to spot the trio, but some care is also needed. You should never scan the sky with optical instruments after the Sun has risen. Brilliant Venus, mag. -3.9, is positioned about 15 degrees above this grouping. On the 10th, Mercury passes just 0.6 degrees south of Regulus. Mercury then passes a mere 3.5 arc minutes north of Mars on the 16th with the waning crescent Moon passing a fraction of a degree south of Mercury on the 18th.
From southern temperate locations, the planet is unsuitably placed for observation.
Algol (β Per) is a bright eclipsing binary system located in the northern constellation of Perseus and one of the best-known variable stars in the sky. Often referred to as the "Demon Star", most of the time it shines at mag. +2.1 but every two days, 20 hours and 49 minutes it suddenly dips in brightness to mag. +3.4, remaining dim for about 10 hours before returning to its original state.
Why the change in brightness? The Algol system consists of at least three-stars (β Per A, β Per B and β Per C) with the orbital plane of Algol A and B directly in line with the Earth. The regular dips in brightness occur when the dimmer B star moves in front of and eclipses the brighter A star. There is also an extra dimension in that a secondary eclipse occurs when the brighter star occults the fainter secondary, resulting in a very small dip in brightness that can be detected with photo-electrical equipment.
NGC 6888, also known as the Crescent Nebula, is a faint emission nebula in the constellation of Cygnus. Although shining at magnitude +7.4, its extremely low surface brightness means a very transparent night or large optics coupled with a nebula filter is required to spot it.
This nebula is located in a dense region of the northern Milky Way. Positioned 2.75 degrees to the northeast is bright star, Sadr (γ Cyg - mag. +2.23), with open cluster, M29, just over a couple of degrees directly east of the nebula. NGC 6888 was discovered by William Herschel on December 15, 1792 and is best seen from northern latitudes during the months of July, August and September. It's object 27 in the Caldwell catalogue.
A total solar eclipse visible from mainland United States takes place on August 21, 2017. On this occasion, the narrow band of totality will first hit land shortly after 10:15 am local time in Oregon. It then crosses through the states of Idaho, Wyoming, Montana, Iowa, Kansas, Nebraska, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina and South Carolina before moving into the Atlantic Ocean. Residents of cities and towns including Salem, Casper, Lincoln, Kansas City, St. Louis, Hopkinsville, Nashville, Columbia and Charleston will experience about a couple of minutes of totality. A partial eclipse is visible from all 50 US states, the width of Canada, Central America, Mexico, the Caribbean islands, northwest Europe and the Chukchi Peninsula in Russia.
NGC 7000 is a giant emission nebula in the constellation of Cygnus that spans 120 x 100 arc minutes of apparent sky. It's one of a number of low surface brightness nebulae surrounding first magnitude star, Deneb (α Cyg - mag. +1.25). Under dark skies this object can be spotted with the naked eye, appearing as a faint hazy patch of light although even the smallest amount of light pollution renders it invisible.
The integrated magnitude of NGC 7000 is probably about +4.5. However, this is misleading due its large size and some parts of the nebula shine no brighter than 12th magnitude. When photographed or imaged, NGC 7000 bears a striking resemblance to the North American continent, hence it's nickname "the North America Nebula".
NGC 7000 was discovered by William Herschel on October 24, 1786 and first photographed by Max Wolf on December 12, 1890. It is 1,600 light-years away and best seen from northern latitudes during the months of July, August and September.
Algol is a bright eclipsing binary system located in the northern constellation of Perseus and one of the best-known variable stars in the sky. Often referred to as the "Demon Star", most of the time it shines at magnitude +2.1 but every two days, 20 hours and 49 minutes it suddenly dips in brightness to mag. +3.4, remaining dim for about 10 hours before returning to its original state.
The Perseid meteor shower is one of the finest annual meteor showers that's almost always reliable and produces a flux of fast and bright meteors. This year's maximum occurs on August 12th at 19 UT, although the 75% illuminated waning gibbous Moon in Pisces will interfere. The best time to look for meteors is during the early morning hours. The Perseids usually produce up to 80 meteors per hour at peak time. In addition, reasonable activity is also expected on the nights of August 11th/12th and August 13th/14th from all Northern Hemisphere locations. Recent analysis by NASA has rated the Perseids as the best meteor shower when it comes to fireballs.
For those at southern locations, the radiant remains low down or never even rises above the horizon. This considerably reduces the amount of visible meteors, although it's possible to spot a few of them coming up above the northern horizon.
When Sir William Herschel observed Mu Cephei in 1783 he described it as a most beautiful object of a very fine deep garnet colour, that's exceptionally striking when compared to nearby white stars. In fact, Mu Cephei is an extremely luminous red supergiant and one of the reddest known stars of all. It may be the largest star visible to the naked eye with an estimated radius of 1.15 billion kilometres (710 million miles) or 1,650 times that of the Sun.
Mercury
Mercury remains visible in the early evening sky during the first part of August. From southern and tropical locations, the elusive planet can be seen low down towards the west-northwest horizon just after sunset. At the beginning of the month it shines at magnitude +0.4 and sets a couple of hours after sunset. By the second week, Mercury will be increasingly difficult to spot as it battles against the bright evening twilight. From mid-latitude northern temperate locations, the planet isn't well placed but can be spotted hugging the western horizon for a few days at the start of month.
On August 2nd, Mercury reaches aphelion when it's 0.467 AU (approx. 69.9 million kilometres or 43.4 million miles) from the Sun. On August 26th, the planet passes through inferior conjunction.
Venus
Venus continues as a brilliant early morning object throughout August. The planet rises around 3 hours before sunrise from northern temperate locations, although considerably less from further south. Its magnitude dims slightly from -4.0 to -3.9 with the illuminated phase increasing from 74% to 83% as the month progresses.
On August 12th, Venus passes 2 degrees south of dwarf planet, Ceres. At magnitude +8.9, Ceres requires at least a pair of binoculars - but more likely a small telescope - to be seen. The waning crescent Moon passes 2 degrees south of the planet on August 19th. On the final day of the month, Venus is just over two degrees west of the large naked eye open cluster, M44, the Praesepe or Beehive.