Perseids shower peaks as summer comet recedes
After our summer twilight, August brings the return of truly dark nights that are rich in bright planets, enlivened by Perseids meteors and, for good measure, witness the retreat of our best comet in years.
The Summer Triangle, formed by the stars Deneb, Altair and Vega, occupies the high southern sky at our map times. It points down to Jupiter which outshines every star and lies in Sagittarius to the right of the Sun’s other gas giant planet, Saturn. The familiar shape of the Plough stands in the north-west as Capella in Auriga twinkles low in the north-east, below and left of the constellation Perseus which, in turn, stands below the “W”-shaped Cassiopeia.
The Perseids shower is already underway as it builds to its expected peak on the 12th. This reliable shower of very swift meteors, many leaving glowing trains or streaks in their wake, yields 80 or more meteors per hour under ideal skies. Expect our best nights to be those of the 11th-12th and 12th-13th while the shower peters out by the 24th.
As their names suggests, Perseid meteors emanate from a radiant point in Perseus which climbs from the northeast at nightfall to lie almost overhead at dawn. Although they appear in every part of the sky, Perseids appear to diverge from the radiant because of the effect of perspective. There is no need for a telescope – just sit back and enjoy the show, though please don’t be disappointed if your meteor rate falls well short of the ideal. Should you find the waning Moon to be obtrusive, try to block it from your vision.
Our summer comet, Comet NEOWISE, is named for NASA’S Wide-field Infrared Survey Explorer space telescope which discovered it in March. It was followed from Earth’s southern hemisphere until it dived into the Sun’s glare in June but there was speculation that it might be a naked-eye object when it remerged in our pre-dawn sky in July. There were also fears that, like the recent comets ATLAS and SWAN, it could fizzle and even disintegrate entirely.
I now regret that I didn’t promote it a month ago, for in early July it rose before the Sun to give us a surprising and stunning cometary display. Easily visible to the unaided eye, it sported an upwards-pointing tail of dust alongside a dim bluish gas tail, best seen in photographs. Before long it became circumpolar, meaning that it remained above our horizon as the Earth’s rotation carried it low across our northern sky from left to right overnight.
Studies suggest that its icy nucleus is 5km wide and that it last approached the Sun some 4,500 years ago. After benefiting from a gravitational boost by the planets, it will not return to our sky for perhaps another 6,800 years. Perihelion, when it was closest to the Sun at 44 million km, came on 3 July and it swept (safely!) 103 million km from the Earth on 23 July.
Now it is an evening object some 30° below the handle of the Plough in the north-west, but it is fading as it recedes from both the Sun and the Earth. Our northern star map plots its path, though I fear we may need binoculars to spot it as August begins, and it may be hard to see at all by the middle of the month. Attempts to anticipate its appearance, though, have been wrong before!
Sunrise/sunset times for Edinburgh change from 05:18/21:19 BST on the 1st to 06:16/20:09 on the 31st. Full moon on the 3rd is followed by last quarter on the 11th, new moon on the 19th and first quarter on the 25th.
Jupiter is obvious low in the south-east as our August nights begin and moves to pass 12° high in the south for Edinburgh at our map times. Weather permitting, it will be doubly hard to miss tomorrow night when it stands 2.5° above the Moon, and the two are close again on the 28th. Saturn is less than a tenth as bright but easily seen 8° to the left of Jupiter.
The two were brightest and closest in July. Now they are beginning to fade and shrink as they recede, but both remain excellent subjects for telescopic study. By midaugust, Jupiter is magnitude -2.6 and its cloud-banded disk is 46 arcseconds wide, while Saturn is magnitude 0.2 and 18 arcseconds across. The Saturnian rings have their northern face tipped 22° towards us and span 41 arcseconds.
The night’s third bright planet, Mars, rises in the east a little before our map times at present and is tracking slowly eastwards in Pisces, below and slightly left of the Square of Pegasus. Brightening from magnitude -1.1 to -1.8. its reddish light is becoming increasingly prominent high in our southern sky before dawn. Telescopically, it swells from 15 to 19 arcseconds making it more worthwhile as a target. Catch it close to the Moon on the morning of the 9th.
Venus now rises more than three hours before the Sun and is a dazzling morning object between the north-east and east. August sees it dim from magnitude -4.4 to -4.2 as its diameter shrinks from 27 to 20 arcseconds and the sunlit portion of its disk changes from 43% to 60%, from crescent to gibbous in phase. Look for it to the right of the slim earthlit Moon on the 16th. Mercury, bright at magnitude -0.8, rises 95 minutes before the Sun on the 1st and may be glimpsed very low in our north-eastern twilight for a few more days before its sinks towards its superior conjunction on the Sun’s far side on the 17th.