July 2015

Sun and Moon

FULL MOON occurs on the 2nd at 04:20 AM. LAST QUARTER (half moon in the morning sky) falls on the 8th. NEW MOON is on the 16th at 16:05 PM. FIRST QUARTER (half moon in the evening sky) falls on the 24th. The second FULL MOON of the month (often referred to as a blue moon, although it is not blue!) occurs on the 31st at 12:43 PM.

On the 16th the young lunar crescent may be visible over the southern Pacific, and under perfect conditions visible from parts of south America. On the 17th July it will be visible worldwide  excluding northern Europe, northern Asia, Canada and Alaska.  It will be visible worldwide apart from the extreme north east of Russia on the 18th July. It will be first visible in South Africa on the 17th.

The Moon will be at perigee (closest approach to Earth) at a distance of about 367 094 km on the 5th at  20:55 PM. On the 21st at 13:03 PM the Moon will be at apogee (furthest from Earth) at a distance of about 404 836 km.

Four meteor showers are active in July. Of these, observing prospects are good for the July Phoenicids. The July Phoenicids meteor shower is active from the 10th July to the 16th July, peaking on the 13th. To view the shower, find a dark spot and look near the constellation of Phoenix. The best time to view the July Phoenicids is from 23:00 PM low in the SE to 05:00 AM when they’ll be in the nearly overhead towards the SE.

Viewing of the following three meteor showers will be hampered by bright moon conditions. The Southern delta Aquariids meteor shower is active from the 21st July to 29th August, peaking on the 29th July. To view the Southern delta Aquariids find a dark spot and look near the constellation of Aquarius for the Southern delta Aquariids radiant. The best time to view the Southrn delta Aquariids is from around 21:30 PM in the east until dawn when they’ll be in the NW.

The alpha Capricornids meteor shower is active from the 15th July to the 25th August peaking on the 30th July. To view the shower look near the constellation of Capricornus for the alpha Capricornids radiant. The best time to view the alpha Capricornids is from around 20:00 PM in the east until 04:00 AM when they’ll be in the west.

The Piscis Australids are active from the 19th July to the 17th August peaking on the 28th July. They are best viewed between 21:30 PM (east) and 05:00 AM (west) looking towards the constellation of Piscis Austrinus (the Southern Fish, not to be confused with Pisces).

Planetary and Other Events – Morning and Evening

Mercury is visible in the early morning for the first half of the month but is then lost in the Sun’s glare. Venus shines as a brilliant evening star all month. Note that Venus and Jupiter make a beautiful close pair during the first week of the month: look early evening towards the West. Mars is visible only at the end of the month just before sunrise very low in the eastern sky.

Jupiter is visible from dusk until around 21:00 PM at the beginning of the month, although by month’s end it will set by 19:30 PM (exact times correspond to Cape Town). Saturn is visible from twilight, setting around 04:45 AM (02:45 AM) at the beginning (end) of the month. Uranus and Neptune are both too faint to observe with the naked eye, but may be seen using binoculars or a small telescope. Uranus is visible from around 01:45 AM (23:45 AM) until daybreak at the beginning (end) of the month. Neptune rises around 22:30 PM at the beginning of the month and is up all night. It rises progressively earlier and ealier each night, until at month’s end it rises around 20:30 PM.

The Evening Sky Stars

The Milky Way is a dominant presence on July evenings, with the brilliant stars of Centaurus nearly overhead, and the Cross just to the south. Marking the southern edge of the Milky Way below the Centaur are the dimmer stars of the Housefly and the Southern Triangle. To the west of Centaurus along the Milky Way is the great ship Argo, with Canopus, second brightest star in the sky, glowing low in the SW. Sirius appears brighter in our sky only because it’s so much closer (9 light years to Canopus’ distance of 313 light years), but Canopus is a supergiant star, 8-9 times as massive as our own Sun, 65 times the Sun’s diameter and 15,000 times as bright. Although the surface temperature of Canopus is ‘only’ 7800 degrees, its atmosphere is heated to about 20 million degrees, meaning plenty of hard radiation for any alien astronaut unfortunate enough to be nearby.

To the east of the Centaur are the stars of the Wolf and the Scorpion, with the Altar just to the south at the edge of the Milky Way. But the thickest part of the Milky Way lies around Sagittarius, the Archer, and the stars of the Scorpion’s sting. In this direction is the centre of our galaxy, and hidden by thick dust clouds is the black hole in the exact centre, 4 million times the mass of our Sun and a bit smaller than the size of Earth’s orbit.

Just north of the Centaur is the tail of Hydra, the giant water snake, with its body extending far into the west almost parallel to the Milky Way. Low in the west is Alphard (Arabic for ‘the solitary one’). Low in the NW are the stars of the Lion, while low in the northeast are the dim stars of the great hero,  Hercules with the delicate semi-circle of the Northern Crown between it and a bright orange Arcturus (the ‘Bear Guard’) low in the north.

Arcturus is the brightest star in Boötes (the Herdsman), which some say is the most ancient constellation in the sky. It looks brighter than any other star in the northern hemisphere, and is an orange giant 37 light years away, 215 times as bright as our sun, and 26 times the Sun’s diameter. Arcturus’ orbit around the centre of the galaxy is quite different from the orbits followed by most stars in our neighbourhood, and it has only 20% as much iron. One possible explanation is that it may originally have been part of a small galaxy that merged with our Milky Way billions of years ago.

The Morning Sky Stars

The Milky Way runs completely around the horizon on July mornings, appearing low in the sky in every direction. That means that when you look overhead you are looking straight from our Milky Way galaxy toward the South Galactic Pole.

Orion the Hunter, with orange Betelguese and blue-white Rigel, is rising in the east. From the northeast, the V-shape of the Bull’s head (with bright Aldebaran as the Bull’s glowing eye) charges Orion. And riding on the back of the Bull is the open cluster of stars called the Pleiades,  which is about 400 light-years away. The Pleiades is also widely known as the Seven Sisters, and known to the Namaquas as “the daughters of the sky god”.

On the low in the ESE we see brilliant Sirius, brightest star in the sky, among the other stars of Orion’s Large Dog, while the Hare scampers between the Dog and the Hunter. The second brightest star in the sky is Canopus, seen in the southeast on July mornings, and marking the Keel of the upside-down Ship Argo. (As most of the constellations were invented in the northern hemisphere, we tend to see them bottom side up.) High in the south is bright Achernar, marking one end of the celestial river Eridanus. The other end is near Rigel about where Orion’s knee would be. Below Achernar in the south are the southern Water Snake and the Toucan, with the Peacock a bit lower in the SW. Alpha Pavonis is actually a pair of hot, luminous blue-white stars about 183 light years away, revolving around each other every 11.75 days. It’s about 450 times as luminous as the Sun.

High in the W are the Crane and the Southern Fish, with its bright star Fomalhaut, with the stars of the Sea Goat making a dim irregular triangle a bit lower in the W. High in the N and NE is the appropriately large constellation of the Whale, reminding us that in a couple of months it will be time for whale-watching again along the Cape coast.

Nicola Loaring   01 July 2015

What's Up

PDF version (two pages, including the full text)
Comments are closed.