The Night sky for July 2010

Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during July 2010.

The nights are gradually beginning to lengthen again, making stargazing a little easier. Arcturus, high in the south in the constellation of Boötes, is bright enough to be mistaken for a planet. To the lower left is the arclet of stars known as the Corona Borealis - the Northern Crown. Up and to the left of that is Hercules, below which lies the mostly empty constellation Ophiuchus, through which the Milky Way passes. The Sun also moves through Ophiuchus, yet it is not one zodiacal houses. Below it, observers at more southern latitudes can see the red star Antares in Scorpius and, to the left, the `teapot' of Saggitarius. Following the tea from the spout leads to the open clusters M6 and M7, while above the lid of the teapot is the Lagoon Nebula, M8. The star Vega is bright in the constellation of Lyra, the Lyre; nearby and lower down, Deneb resides in Cygnus, the Swan; Altair rises about an hour after sunset in Aquila, the Eagle. These three stars complete the Summer Triangle, within which lies a rich region for observations with binoculars or telescope.

The Planets

• Jupiter rises soon after midnight in late July, and shines high in the dawn sky at magnitude -2.7. One of its dark bands, the south equatorial belt, has recently disappeared, while the nearby Great Red Spot has become darker and more prominent than it has been in several years.
• Saturn is visible to the south-west of Virgo and Leo just before sunset. It's magnitude is +1.1, with an angular diameter of 17" and rings out to 38". The plane of the rings is now about 3 degrees from edge-on to our line of sight and increasing. A number of its moons, the brightest of which is Titan, are visible using a telescope.
• Mercury is an evening object, appearing about 6 degrees above the horizon in the west-north-west, half an hour after sunset. It is visible at magnitude -0.5 in the middle of the month, dropping to +0.1 by the end. Its angular size increases from about 5 to 7" over the same period. On the 27th it passes 0.3 degrees south of the star Regulus in Leo, the Lion.
• Mars is at magnitude +1.4, decreasing through the month. It appears in the south-west evening sky to the lower left of the star Denebola in Leo's tail, between Regulus in Leo and Spica in Virgo, the Virgin. Its angular size is 5".
• Venus dominates as the Evening Star at a magnitude of about -4.1 throughout the month. It is approaching the Earth as its illuminated surface area decreases, keeping its brightness almost constant.

Highlights

• The thin crescent Moon will be just to right of the Pleiades cluster in the pre-dawn sky of the 8th. Earthshine - sunlight reflected off the Earth's clouds - should be visible on the shadowed part of the Moon's surface. The brighter stars of the Pleiades will be discernible with binoculars.
• Looking with binoculars up the right side of the keystone - the four stars in a trapezium at the heart of Hercules - will lead to the globular cluster M13, the brightest globular cluster in the northern sky. Globular clusters are small, spherical concentrations of stars in the galactic halo which surrounds the centre of the Milky Way. They are as old as the galaxy, and each appears as a fuzzy ball which contains roughly a million stars. Harlow Shapley plotted the positions of globular clusters, finding them to be spherically distributed about the galactic centre. From this, he inferred the distance from the Sun to the centre of the Milky Way to be 27,000 light years.
• Saturn, Mars and Venus line up on the evening of the 15th, above and to the left of the Moon.
• On the 20th, the Moon's terminator (edge of illumination) will be close to two of its craters, Tycho and Copernicus, allowing them to be seen clearly through binoculars. Tycho is 85 km wide and 5 km deep, lying in the Southern Lunar Highlands near the bottom of the Moon. It is thought to have been formed 108 million years ago by a fragment of one of the Baptistina family of asteroids, another of which may have created the Earth's Chicxulub crater and wiped out the dinosaurs 65 million years ago. At full Moon, radial debris lines emanating from Tycho are visible. Copernicus is around 800 million years old and lies higher than Tycho, in the eastern part of Oceanus Procolarum, beyond the curving Appenine Mountains. It is 93 km wide and 4 km deep, with terraced, rather than sheer, sides.
• The largest object in the Asteroid Belt, the dwarf planet Ceres, can be seen in the lower part of Ophiuchus. Although normally washed out by the light from the Milky Way, in July it passes in front a cloud of dust and gas called Barnard 78. At magnitude +7, it can be viewed with binoculars below the star Theta Ophiuchi.
• Mars and Saturn are close together on the 31st, above and to the left of Venus.
• Uranus can easily be found to the right of Jupiter with binoculars during July, and, at magnitude +5.8, may be just visible to the naked eye under very dark skies. Two objects of similar brightness will be visible near one another, the right-hand one being Uranus. Its turquoise disc can be seen through a small telescope. Jupiter will move noticeably further to the left of Uranus during the month.

The Southern Night Sky

John Field from the Carter Observatory in New Zealand tells us some of the highlights that can be seen in the southern night sky during July 2010.

The smallest constellation, Crux - the Southern Cross - is high to the south in the winter sky, with its two pointer stars to the left. Crux was known to the ancient Greeks and was considered part of the larger constellation Centaurus, but precession of the equinoxes changed Earth's orientation and pushed it south over the centuries. It is Te Punga - the anchor - to the Māori, and represents a possum in a tree to Australian Aborigines. The brightest star in Crux, Alpha Crucis, is actually a double star system, resolvable with a telescope. The second-brightest star, Beta Crucis, is also in a binary system. The Jewel Box, a hazy patch of light nearby, can be resolved into a beautiful open cluster using binoculars or a telescope. John Herschel made its first telescopic observation. The southern celestial pole is located near the halfway point of a line pointing upwards from Crux towards the bright star Achernar. The Magellanic Clouds, two more hazy patches about three quarters of the way along the line, can be seen unaided in a dark sky. The Large Magellanic Cloud (LMC) lies about 160,000 light years from us and is around 14,000 light years in length; the Small Magellanic Cloud (SMC) is about 200,000 light years away and 7,000 light years long. These dwarf galaxies interact with each other and orbit the Milky Way. The LMC has a barred spiral structure, with its own star clusters and emission nebulae, the brightest of which is called the Tarantula. This is distinguishable on the left side of the LMC using a telescope, and has an estimated mass of 450,000 Solar Masses. It is the most active known star-forming region in the Local Group of galaxies, and may one day become a globular cluster. If it were as close to us as is the Orion Nebula in our own galaxy, it would be bright enough to cast visible shadows on the Earth at night. At the centre of the Tarantula Nebula is a compact star cluster, R136, containing many giant stars. The brightest supernova since the invention of the telescope, Supernova 1987A, was observed near this region by astronomers in New Zealand and Chile in 1987.

The Planets

• Venus, Mars and Saturn are high in the north.
• Venus will be close to Regulus on the 10th.
• Mars moves from Leo to Virgo during the month, and approaches Saturn in the night sky.
• Jupiter is visible in the early morning, but rises at 10pm by the end of the month.

Highlights

• The Milky Way stretches along the southern horizon before sunrise. From southern Australia's Blue Mountains, the Milky Way forms a spectacular ring around the horizon, revealing the structure of our galaxy.