New Horizons to be first at dwarf planet Pluto

The maps show the sky at 01:00 BST on the 1st, midnight on the 16th and 23:00 on 31st. (Click on map to englarge)

The maps show the sky at 01:00 BST on the 1st, midnight on the 16th and 23:00 on 31st. (Click on map to englarge)

Since our nights are still awash with summer twilight, we may be excused if our attention this month turns to Pluto as it is visited by a spacecraft for the first time.

Pluto was still regarded as a the solar system’s ninth planet when NASA’s New Horizons mission launched in 2006, but it was officially reclassified as a dwarf planet later that year. We now recognise it as one of several icy worlds, and not even the largest, in the Kuiper Belt of such objects beyond the orbit of Neptune.

Pluto is 2,368 km wide and has a system of five moons, two of them only discovered while New Horizons has been en route. The largest moon, Charon, is 1,207 km across and, like Pluto itself, will block the probe’s signal briefly as New Horizons zooms through the system at a relative speed of almost 14 km per second on the 14th. The closest approach to Pluto is due at 12:50 BST at a range of some 12,500 km from Pluto and 4,772 million km from the Earth. It will not hang around, though, since this is a flyby mission and the probe will speed onwards, perhaps to encounter another still-to-be-identified Kuiper Belt world before the end of this decade.

A conjunction of an altogether different type is gracing our western evening sky as July begins. The two most conspicuous planets have been converging over recent weeks and stand at their closest on the 1st when the brilliant Venus passes within 21 arcminutes, or two-thirds of a Moon’s breadth, of Jupiter.

On that evening, Venus is just below and left of Jupiter and sinks from 14° high in the west at sunset to set in the west-north-west about 100 minutes later. Shining at magnitude -4.4 from a distance of 76 million km, Venus appears 33 arcseconds in diameter and 33% illuminated, its dazzling crescent visible through a small telescope or even binoculars. Jupiter, 911 million km away on that evening, is 32 arcseconds wide but only one eleventh as bright at magnitude -1.8.

Over the coming days, Venus slips to the left with respect to Jupiter as both planets drop lower into the twilight. By July 15, Venus is only 7° high at sunset and sets within the hour, while Jupiter is 5° to its right and slightly higher, but unlikely to be seen without binoculars and a clear horizon. By then, too, Venus has closed to 61 million km and its crescent is taller but narrower, 41 arcseconds and 22% sunlit.

It is perhaps surprising that the Earth reaches aphelion, the farthest point from the Sun in its annual orbit, on the 6th. We are then 152,093,481 km from the Sun, 4,997,277 km further away than we were at perihelion on January 4.

Sunrise/sunset times for Edinburgh change from 04:31/22:01 BST on the 1st to 05:14/21:23 on the 31st. The Sun tracks 5° southwards during July to bring the return of nautical darkness for Edinburgh from the night July 11/12. By the last night of the month this official measure of darkness lasts for almost four hours.

The Moon is full on the 2nd, at last quarter on the 8th, new on the 16th, at first quarter on the 24th and full again on the 31st – there is a notion, sadly mistaken, that a second full moon in a month should be termed a “blue moon”.

The white star Vega climbs from high in the east at nightfall to dominate our high southern sky at the star map times. It stands at a distance of 25 light years, being the third brightest star ever seen in Scotland’s night sky after Sirius, which is currently out of sight, and Arcturus in Bootes which stands low in the west by the map times.

Vega is twice as massive as our Sun and 40 times more luminous but only one tenth as old. It has an extensive disk of dusty material, although observations hinting that this contained a planet appear not to be supported my more recent studies.

Vega is the leader of the small box-shaped constellation of Lyra the Lyre, representing a small harp from classical times. It is also the brightest star in the Summer Triangle that includes Deneb in Cygnus, high in the east at the map times, and Altair in Aquila, in the middle of our south-east. Far to the south of Vega is the so-called Teapot in Sagittarius which seems to be pouring to the right as it sits on Scotland’s southern horizon. We have much clearer views of this region of sky, rich in stars and star clusters, if we view them higher above the horizon under darker skies further south.

The red supergiant Antares in Scorpius glowers to the right of the Teapot and lies 13° below-left of Saturn which is twice as bright at magnitude 0.3 to 0.4. Indeed, after the Moon, it is the brightest object low down in the south at nightfall, moving to the south-west by our map times and setting less than two hours later. Currently creeping westwards in eastern Libra, it shows an 18 arcseconds disk through a telescope, set within glorious rings that stretch across 41 arcseconds and have their north face inclined towards us at 24°. Catch the Moon to the right of Saturn on the 25th and to its left a day later.

Of the other planets, Mars has yet to emerge from the Sun’s glare while Mercury hides low in our bright morning twilight as it moves towards superior conjunction on the Sun’s far side on the 23rd.

Alan Pickup

This is a slightly-revised version of Alan’s article published in The Scotsman on June 30th 2015, with thanks to the newspaper for permission to republish here.