Venus, Earth’s hellish twin, at its highest for eight years
The current evening apparition of Venus reaches its climax during March when it stands higher at sunset than it has for eight years. While Orion remains an imposing sight in the south at nightfall, it is falling lower towards the west by our star map times.
Attention, though, is still fixed on the red super giant star Betelgeuse at Orion’s shoulder, see map, which has been suffering from the deepest plunge in its brightness ever seen. For much of February, the star has been close to the magnitude 1.6 of Bellatrix, the star at Orion’s other shoulder, and more than a magnitude fainter than usual. Meantime, images of the star’s surface using the Europe’s VLT (Very Large Telescope) in Chile show an apparent change of shape, perhaps due to a giant star-spot or to obscuration by a cloud of dust ejected from the surface.
Studies of periodicities in Betelgeuse’s lesser pulsations over the years had led some analysts to suggest that 21 February might bring a turning point in its fortunes. In fact, measurements show that its brightness decline may have reversed a few days earlier than this, though it has far to go to regain its former prominence.
March sees the Sun climb 12° northwards to cross the sky’s equator at 03:50 GMT on the 20th, marking our vernal or spring equinox. Edinburgh’s sunrise/sunset times change from 07:03/17:48 GMT on the 1st to 06:45/19:50 BST (05:45/18:50 GMT) on the 31st, after we set our clocks forward on the 29th. The Moon reaches first quarter on the 2nd, is full on the 9th, at last quarter on the 16th and new on the 24th.
Our charts show the Sickle of Leo high on the meridian as the Plough is approaching the zenith from the east and the Pleiades star cluster is sinking in the west where it lies 7° above-right of the Moon on the 1st. Between Gemini and Leo lies the dim constellation of Cancer and its Praesepe or Beehive star cluster at a distance of 577 light years. Praesepe’s individual stars may be spotted through binoculars but will be swamped by the Moon when it lies just above-left of the cluster on the evening of the 6th.
Venus, brilliant and unmistakable at magnitude -4.2, stands 36° high in Edinburgh’s south-west at sunset on the 1st and sets in the west-northwest at 22:22 GMT. Tracking east-north-eastwards against the stars, it brightens to magnitude -4.4, slides to the left of Hamal in Aries, and approaches to within 3° or 6 Moonbreadths of the Pleiades by the evening of the 31st. Its distance drops from 133 million to 98 million km, while, viewed telescopically, it swells from 19 to 25 arcseconds in diameter and evolves from gibbous to crescent in phase as the dazzlinglylit portion of its disk changes from 63 per cent to 47 per cent.
The slim young Moon is below Venus on the 27th and near Venus and the Pleiades (again) on the 28th. Venus is furthest from the Sun (46°) on the 24th – its so-called greatest elongation – but is actually highest at sunset two days later at 38.6°.
There is a curious correlation between the orbital periods of Venus and the Earth in that eight orbits of the Earth take 2922 days while 13 orbits of Venus total 2921 days. This means that this apparition of Venus is an almost exact dayfor-day (or night-for-night) replay of the one it enjoyed eight years ago, in 2012, and we will not see Venus so high again in the evening until 2028.
In some respects, Venus is almost the twin of the Earth since it is 95 per cent as wide and has 81 per cent of its mass. However, if the ancient Romans had been aware of its true nature, they might not have chosen to name it for their goddess of love and beauty. The planet has a dense atmosphere of carbon dioxide and a surface as close to hellish as we can imagine. Any astronaut brave enough to descend through its opaque clouds, mainly of sulphuric acid, would find themselves roasted at 460°C and compressed under an atmospheric pressure 92 times that at the Earth’s surface, comparable with an ocean depth of 900m.
Venus spins backwards on its axis in 243 earth-days and much of its surface, only really “visible” to orbiting probes via radar, appears to be blanketed by plains of lava with no sign of the plate tectonics that drive so much of our planet’s geology. There are more volcanoes than on the Earth and suggestions that a handful may have been erupting as we “watch”. There are impact craters, too, but few small ones because of the shielding effect of the atmosphere.
The Roman heritage lives on in that most of its surface features are names for real or mythological women – one notable exception is that its highest mountain, Maxwell Montes, is named for Edinburgh’s James Clerk Maxwell.
Mercury stands furthest west (28°) of the Sun on the 24th but stays hidden from our view in the dawn twilight. The other bright planets are inconveniently low in our south-south-eastern sky before dawn. The brightest, magnitude -2.0 Jupiter, stands 8° high one hour before sunrise and is about 7° to the right of the fainter Saturn, magnitude 0.7. Fainter still is Mars (magnitude 1.0 to 0.8) which lies 10° to the right of Jupiter on the 1st but moves to pass 0.7° south of Jupiter on the 20th and 0.9° south of Saturn as the month ends. Catch the waning Moon below-right of Mars and Jupiter on the 18th.