Jupiter and Saturn in their closest meeting since 1623

Alan Pickup

Author: Alan Pickup
The maps show the sky at 21:00 GMT on the 1st, 20:00 on the 16th and 19:00 on the 31st. An arrow depicts the motion of Mars.

A month that brings the superb Geminids meteor shower under moonless skies, also sees the two largest planets converge for their closest conjunction since the summer of 1623. Back then, though, Jupiter and Saturn were too close to the Sun to be spotted just after sunset. We would need to set our time machine for an early spring morning in 1226 to catch the two planets closer together than they will be this December.

The iconic constellation of Orion, surely the most recognisable one of all, lies opposite the Sun this month so that it dominates our southern sky in the middle of the night. Above it as it climbs in the east this evening is Taurus the Bull whose eye is marked by the orange giant star Aldebaran. The Pleiades star cluster is nearby, but swamped by the bright Moon as the month begins. Once the Moon is out of the way, use binoculars to view Orion’s Sword hanging below his Belt and spot the Orion Nebula, a hazy cauldron of gas and dust in which new generations of stars and planets are forming as we watch from a distance of 1,350 light years.

The Geminids shower is expected to peak at about 01:00 on the night of the 13th-14th when, under ideal conditions, 100 or more meteors per hour might be seen streaming away a radiant that lies close to the star Castor in Gemini. This point lies above and left of Orion as it climbs from low in the north-east at nightfall to pass more than 60° high in the south at 02:00. All of which means that, weather permitting, the early morning of the 14th is a time not to be missed if we want to marvel at 2020’s best meteor display in full flow.

In fact, the shower lasts from the 4th to the 17th and respectable numbers of its slow meteors arrive between the 12th and the 15th, so there is still hope even if the peak night is clouded out. Remember, too, that we do not need to be facing Gemini to see them – Geminids appear in all parts of the sky and it is just perspective that makes their parallel paths into our upper atmosphere appear to diverge from that direction as they disintegrate at 35km per second.

Jupiter and Saturn have featured low down in our southern night sky for several months, but are now slipping even lower into the south-west as the night begins and set about 90 minutes before our star map times. Even so, and providing we can engineer a clear view in that direction, Jupiter is outstanding at magnitude -2.0 while Saturn is a tenth as bright at magnitude 0.6 and, currently, some 2° above and to Jupiter’s left.

Both planets lie 7° to the right of the slender earthlit Moon on the 17th as they edge slowly eastwards (to the left) against the stars to cross from the constellation Sagittarius into Capricornus. Jupiter is moving more quickly, though, and overtakes Saturn on the 21st, passing only 6 arcminutes, or one fifth of a Moon’s breadth, south of Saturn. By then, they stand 8° high one hour after sunset and will make an impressive sight in the same telescopic field of view, with Saturn and its stunning rings above Jupiter and its moons.

While they appear so close together, they are at very different distances – Jupiter being 886m km away and Saturn another 734m km beyond Jupiter. At the time, the cloud-banded Jovian disk appears 33 arcseconds wide while Saturn’s rings are tipped 21° earthwards and span 35 arcseconds around its 15 arcseconds disk. By Hogmanay, their separation has grown to 1.1° and Jupiter is only 5° high one hour after sunset.

The 21st also brings our winter solstice, when the Sun reaches its furthest south in the sky at 10:02 GMT. The sunrise/sunset times for Edinburgh change from 08:20/15:44 on the 1st, to 08:42/15:40 on the 21st and 08:44/15:48 on the 31st. The Moon is at last quarter on the 8th, new moon on the 14th, first quarter on the 21st and full moon on the 30th.

For the second time in 17 months, and coronavirus permitting, observers in Chile and Argentina have the chance to experience a total eclipse of the Sun on the 14th, though this time the narrow path along which totality is visible sweeps about 1,000 km further south as the Moon’s shadow speeds from the Pacific to the Atlantic. Surrounding areas enjoy a partial solar eclipse, but Europe gets nothing.

The dim and distant planets Uranus and Neptune may be glimpsed in our evening sky but cannot compare with the orange light of Mars. True, the latter more than halves in brightness this month, from magnitude -1.1 to -0.2, as its distance grows from 96 million to 134 million km, but it still outshines every star in the sky until Sirius rises in the south-east one hour after our map times.

Now moving eastwards in Pisces, Mars is unmistakable in the south-east at nightfall, passes 41° high across Edinburgh’s meridian shortly before the map times and then sinks to our western horizon in the early hours. Telescopically, its disk appears to shrink from 15 to 10 arcseconds which will make it harder to trace the evolution of a Martian dust storm that was first spotted on 12 November and has been photographed by several amateur astronomers, including from Scotland. Catch Mars above the gibbous Moon on the 23rd.

Venus continues as a brilliant (magnitude -4.0) morning object, though it is sinking lower as it moves towards the Sun’s far side. The planet rises for Edinburgh at 05:32 on the 1st and at 07:11 on the 31st, while its altitude in the south-east at sunrise halves from 16° to 8° during the period. It lies 7° below-left of the waning earthlit Moon on the 12th. Mercury sweeps around the Sun’s far side on the 20th and is hidden from view.

Diary for 2020 December

  • 5th 02h Moon 2.7° N of Praesepe
  • 6th 17h Moon 5° N of Regulus
  • 8th 01h Last quarter
  • 10th 12h Moon 7° N of Spica
  • 12th 21h Moon 0.8° N of Venus
  • 14th 01h Peak of Geminids meteor shower
  • 14th 16h New moon and total solar eclipse
  • 17th 04h Moon 2.9° S of Jupiter
  • 17th 05h Moon 3.1° S of Saturn
  • 20th 03h Mercury in superior conjunction
  • 21st 10:02 Winter solstice
  • 21st 14h Jupiter 0.1° S of Saturn
  • 22nd 00h First quarter
  • 22nd 09h Peak of Ursids meteor shower
  • 23rd 01h Venus 6° N of Antares
  • 23rd 18h Moon 6° S of Mars
  • 26th 21h Moon 6° S of Pleiades
  • 27th 21h Moon 5° N of Aldebaran
  • 30th 03h Full moon

This is an extended version, with added diary, of Alan’s article published in The Scotsman on November 30 2020, with thanks to the newspaper for permission to republish here.