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Mercury

Mercury reaches greatest elongation on July 12th, when it moves out to 26.4 degrees east of the Sun. The planet is visible just after sunset low down above the western horizon in evening twilight. On July 3rd and 4th, Mercury passes within a degree of sprawling open cluster M44, also known as the Praesepe or Beehive cluster. This cluster can be seen with the naked eye under dark skies, but not against the twilight sky. However, it should be visible with binoculars. From northern temperate locations, Mercury rapidly fades from view after greatest elongation and by the third week of the month will be lost to the bright twilight. For those further south, the planets visibility period is better still.

From July 1st to 15th, Mercury fades in brightness from magnitude -0.1 to 0.5. Much brighter Venus, mag. -4.2, appears about 15 degrees above Mercury. On July 14th, the thin waxing crescent Moon passes 2 degrees north of Mercury.

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 brightness change. 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.

Algol System (credit:- Dept. of Physics and Astronomy - Univ. of Tennessee at Knoxville)

Vesta the brightest asteroid or minor planet is currently easily visible with binoculars and small telescopes as it reaches opposition on June 20th. In addition, this opposition is extremely favourable and keen eyed observers, from dark locations, may even be able to glimpse the asteroid with the naked eye. Vesta shines at magnitude +5.3 for a few days either side of June 20th. It stays above magnitude +6.0 until the middle of July, and remains bright enough to be easily followed for the next few months with binoculars and small scopes.

Vesta as imaged by the NASA Dawn spacecraft (credit:- NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)

Discovery

Vesta was the fourth asteroid to be discovered on March 29, 1807 by German astronomer Heinrich Wilhelm Olbers. With a diameter 530 kilometres (330 miles) it's the third largest body in the main asteroid belt after Ceres and Pallas. However, Vesta is the brightest asteroid because it approaches closer to Earth and has a higher albedo or reflective surface.

Saturn is now at its best for the year and on June 27th opposition is reached. Currently located in Sagittarius the planet is visible all night long, but with a declination of -22 degrees is much better placed from southern and tropical locations. For example, Saturn reaches a maximum altitude of 78 degrees and is visible for over 13 hours from Sydney, Australia. Whereas from New York City, it climbs just 28 degrees high with a visibility period lasting 9 hours.

Saturn imaged by the Hubble Space Telescope (credit:- NASA, ESA, and The Hubble Heritage Team (STScI/AURA))

At opposition, Saturn shines at magnitude 0.0 and is located 9.049 AU (approx. 1,354 million kilometres or 841 million miles) from Earth. Of course, the spectacular rings are its most famous feature and even a small telescope will show them. The rings are currently displayed wide open with a 26.5 degrees tilt, and last October they were at their widest possible, an inclination of 27 degrees. In addition, a handful of Saturn's moons are also visible. The largest and brightest is eighth magnitude Titan, which can be seen with binoculars. A 100mm (4-inch) refractor will show others, including Rhea, Tethys and Dione. At this time of year the moons are also at their brightest.

Mercury

Mercury reaches superior conjunction on the far side of the Sun on June 6th. The fast moving planet then moves rapidly out from the Sun to become an early evening object, visible low down above the western horizon, during the second half of the month. It subsequently improves in altitude each evening, although fading in brightness at the same time. However from northern temperate locations, Mercury has to battle against the bright evening twilight and therefore best glimpsed towards the end of month. On June 15th, the planet shines at magnitude -1.2 but by months end it will have dropped to magnitude -0.2. Mercury then sets up to 90 minutes after the Sun. Much brighter Venus, is located in the same region of sky, although considerably higher above the horizon.

On June 25th, Mercury passes 5 degrees south of Pollux (mag. +1.1). At magnitude +0.4, Mercury will be the brighter of the pair.

Venus

Venus continues to illuminate the western sky after sunset. The brilliant "evening star" is visible as soon as it's dark enough and dominates the view. Brighter than all other planets, Venus continues to increase in brightness, improving from magnitude -3.9 to -4.1 as the month progresses. During this time, its angular size increases slightly from 13 to 15 arc seconds. Venus begins June in Gemini, close to Pollux (mag. +1.1), before moving into Cancer on the June 11th. Later on June 19th and 20th, it passes near to M44, also know as the Praesepe or Beehive Cluster. Throughout the month the planet sets between 2.5 to 3 hours after the Sun.

On June 16th, the thin waxing crescent Moon passes a couple of degrees south of Venus, providing pleasant early evening viewing.

Venus and the crescent Moon, as seen from mid-latitude northern locations, just after sunset on June 16, 2018 (credit:- freestarcharts)

M16, also known as the Eagle Nebula, is a young open cluster of stars embedded within an extremely large cloud of interstellar gas and dust in the constellation of Serpens (Cauda). It's located in the next inner spiral arm of the Milky Way, 7,000 light-years distant. The emission part of the nebula or HII region is catalogued as IC 4703 and is an active star-forming region, which has already created a significant cluster of young stars. The cluster itself lies at the heart of the Eagle Nebula and is known as NGC 6611. M16 was discovered by Philippe Loys de Chéseaux in 1745-6, but Charles Messier was the first to record the associated nebulosity on June 3, 1764.

The constellation of Serpens is faint but unique as its split into two separate sections. One half, named Serpens Caput, lies to the west of Ophiuchus and the other half, Serpens Cauda, lies on the eastern side of Ophiuchus. At the very southern tip of Serpens Cauda close to the Scutum and Sagittarius border is M16. It can be found 2.5 degrees west of γ Sct (mag. +4.7) and a few degrees north of the Omega Nebula (M17), M18 and the Sagittarius Star Cloud (M24). This beautifully rich area of the sky is a delight to scan with binoculars.

The Eagle Nebula was immortalised in 1995 when imaged several times by the Hubble Space Telescope. The resulting iconic photograph, titled the Pillars of Creation, showed three magnificent columns of interstellar gas and dust displayed in sensational detail.

M16 The Eagle Nebula showing the Pillars of Creation star forming region (credit:- NASA, ESA, STScI, J. Hester and P. Scowen (Arizona State University))

M24, also known as the Sagittarius Star Cloud, is a large naked eye expanse of stars, clusters, nebulosity and other objects located in Sagittarius. At mag. +4.6 and covering 1.5 degrees of sky, it's visible to the naked eye as a large detached part of the Milky Way. The object is a fantastic sight in binoculars and small telescopes. It's claimed that M24 has the densest concentration of individual stars visible, around a thousand, in a single binocular field of view.

The Sagittarius Star Cloud is not a true deep sky object but results from a rare alignment between the Earth and the centre of our galaxy. We would expect this region to be packed with interstellar dust, however by chance we are looking through a gap in the dust. As a result, many thousands of distant stars, clusters and nebulae are visible that would otherwise be obscured. Spatially, M24 covers a volume up to 16,000 light-years deep.

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 brightness change. 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.

Algol System (credit:- Dept. of Physics and Astronomy - Univ. of Tennessee at Knoxville)

Jupiter, in Libra, is a dazzling object this month and on May 9th the planet is at its best for the year when opposition is reached. On this day, it will shine at mag. -2.5 and visible all night long. It rises in the east at sunset, reaches its highest point in the sky around midnight before setting in the west at sunrise. The planet is unmistakable to the naked eye and easily brighter than any night time star. Due to its declination of -16 degrees, Jupiter is currently better seen from southern rather than northern latitudes.

Jupiter as imaged by Hubble Space Telescope on April 21, 2014 (credit:- NASA, ESA, and The Hubble Heritage Team (STScI/AURA))

At opposition, Jupiter is located 4.400 AU (658.2 million kilometers or 409 million miles) from Earth. Its apparent size spans an impressive 44.8 arc seconds. Of all other planets, only Venus can exhibit a greater apparent size than Jupiter.