02 Jul Whats Up – July 2024

Sun and Moon

The New Moon occurs on the 6^th of July at 00h57 and the First Quarter falls on the 14^th of July at 00h48. The Full Moon occurs on the 21^st of July at 12h17 and the Last Quarter Moon falls on the 28^th of July at 04h51.

The Moon will be at perigee (closest approach to Earth) at a distance of about 364 917 km on the 24^th of July at 07h41. On the 12^th of July at 10h11, the Moon will be at apogee (furthest from Earth) at a distance of about 404 362 km.

Planetary and Other Events – Morning and Evening

Mercury, the smallest planet in our solar system, can be observed just after sunset in the west-northwest. It is near the Moon and the Beehive Cluster on the 7^th of July. In the last week of July, Venus can be observed low above the horizon, also just after sunset and in the west-northwest. It will be visible as the “evening star” for the next months. This is a bit of a misnomer, though: As a planet, Venus is not a star; it is not producing its own light. It is named after the Roman goddess of love. But with temperatures exceeding 400 degrees Celsius and an atmospheric pressure about 90 times that on Earth, it is definitely not a lovely place!

Saturn is visible throughout this month from about midnight, rising in the east. Jupiter and Mars can be observed before sunrise in the north and north-northeast, respectively.

Ceres, the inner dwarf planet, reaches opposition on 5^th of July and therefore best positioned for observation. The dwarf planet Pluto reaches opposition on the 23rdof July and is therefore best positioned for observation. While you may spot Ceres through a pair of binoculars, you need a large telescope to see Pluto.

Four meteor showers are active in July, and observing prospects are good for all of them. The Southern delta-Aquariids meteor shower is active from the 12^th of July to the 23^rd of August, peaking on the 31^st of 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 point in the sky from which the meteor shower seems to originate). The best time to view the Southern delta-Aquariids is from around 22h00 in the east until 05h00 when they’ll be in the NW. The alpha-Capricornids meteor shower is active from the 3^rd of July to the 15^th of August, peaking on the 31^stof 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 20h00 in the east until 04h00 when they’ll be in the west.

The Piscis Australids are active from the 15^th of July to the 10^th of August, peaking on the 28^th of July. They are best viewed between 21h30 (east) and 05h00 (west) looking towards the constellation of Piscis Austrinus (the Southern Fish, not to be confused with Pisces).The eta-Eridanids are active from the 31^st of July to the 19^thof August, peaking on the 8^th of August. They can best be viewed between 01h00 and 05h30, looking toward the constellation of Eridanus in the east.

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 nightsky, 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 about one sixth 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 Betelgeuse 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”.

Low in the ESE we see brilliant Sirius, brightest star in the nightsky, 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.

Sivuyile Manxoyi
sivuyile@saao.ac.za
X: @rassivuyile

The evening sky over Cape Town

The evening sky over Johannesburg