Two brightest planets in closest meeting for 14 years
Our usual highlight for August is the return of the prolific and reliable Perseids meteor shower. Unfortunately, meteor-watchers have to contend with moonlight this year and it is just as well that we have other highlights as compensation. Foremost among them is the closest conjunction between Venus and Jupiter, the two brightest planets, since 2000 though they are low down in our morning twilight. Mars and Saturn rendezvous, too, and we have our best supermoon of the year.
Sunrise/sunset times for Edinburgh change from 05:17/21:20 BST on the 1st to 06:15/20:10 on the 31st. The spell of nautical twilight at dusk and dawn shrinks from 121 to 89 minutes. The Moon is at first quarter on the 4th, full on the 10th (see below), at last quarter on the 17th and new on the 25th.
The term supermoon has gained currency in recent years to describe a full moon that occurs when the Moon is near its closest in its monthly orbit. At such times, it can appear 7% wider and 15% brighter than an average full moon. In my view, the enhancement is barely perceptible to the eye and is less impressive than the illusion that always makes the Moon appear larger when it is near the horizon. As the media have discovered, though, supermoons provide a good excuse to feature attractive images of the Moon against a variety of landscapes, and if this encourages more people into astrophotography, so much the better.
This month, the Moon is full at 19:10 BST on the 10th, less than 30 minutes after its closest point (perigee) for the whole of 2014. On that evening, the supermoon is already 4° high in the east-south-east as the Sun sets for Edinburgh, so judge (and photograph?) for yourself.
Venus is brilliant at magnitude -3.9 as a morning star. Rising at Edinburgh’s north-eastern horizon at 03:10 BST on the 1st, it stands 15° high in the east-north-east at sunrise. By the 31st, it rises at 04:42 and is 12° high at sunrise. Between these dates it is caught and passed by Jupiter which emerges from the twilight below and to Venus’ left on about the 7th and stands only 0.2° below Venus before dawn on the 18th.
Jupiter is magnitude -1.8, one seventh as bright as Venus, but still outshines every star so the conjunction is a spectacular one, albeit at an inconvenient time of the night. In fact, the event occurs less than a degree south-west of the Praesepe or Beehive star cluster in Cancer, but this may be hard to spot in the twilight. Before dawn on the 23rd, the two planets lie to the left of the waning and brightly earthlit Moon. By the month’s end, Jupiter rises by 03:25 and stands 13° above-right of Venus.
Mars and Saturn have set by the map times but stand low in the south-west as our evening twilight fades. On the 1st, Mars is magnitude 0.4 and lies 10° to the left of Spica in Virgo. Saturn, only a little dimmer at magnitude 0.5 in Libra, is 13° to Mars’ left, and slightly higher. Look for the Moon to the right of Mars on the 2nd, between Mars and Saturn on the 3rd and to the left of Saturn on the 4th. Mars, meanwhile, tracks eastwards to cross from Virgo to Libra on the 10th and pass 3.5° below Saturn on the 24th. By the 31st, both planets have faded to magnitude 0.6, and Mars lies 5° below-left of Saturn with the Moon between them again and very close to Saturn.
After passing around the Sun’s far side on the 8th, Mercury is too low to be seen in our evening twilight.
Our chart depicts the bright stars Deneb, Vega and Altair high in our southern sky where they form the Summer Triangle. The centre of our Milky Way galaxy lies in Sagittarius on the south-south-western horizon but the Milky Way itself flows northwards through Aquila and Cygnus before tumbling down through Cepheus, Cassiopeia and Perseus in the north-north-east.
Use binoculars to seek out the star Mu Cephei high above familiar “W” of Cassiopeia. Dubbed the Garnet star by Sir William Herschel, Mu is one of the reddest stars we know and pulsates semi-regularly between magnitude 3.4 and 5.1. Some 6,000 light years away, it is so large that it would extend beyond the orbit of Saturn if it replaced the Sun and is sure to explode as a supernova within a few million years.
The Perseids are due to peak in the middle of the night on 12-13th August when we might have been able to glimpse more than 80 meteors per hour under ideal conditions. As it is, bright moonlight will ensure that meteor counts are well down, though we can still expect some impressive bright meteors that leave persistent glowing streaks, called trains, in their wake.
Decent rates may be seen from perhaps 10-15th August and, in fact, the shower is already underway as the Earth takes from 23 July to 20 August to traverse the stream of Perseid meteoroid particles laid down by Comet Swift-Tuttle. It is only appropriate that the resulting meteors are swift, too, as they disintegrate in the upper atmosphere at 59 km per second. Although they move in parallel through space, perspective means that they appear to diverge from a radiant point in Perseus, plotted on our northern star map below Cassiopeia. That point climbs through the north-east overnight to approach the zenith by dawn, but remember that the meteors can appear in any part of the sky and not just towards the radiant.