Those of you going out late in the day for an evening stroll or just enjoying the spring smells in your garden may have noticed a bright white beacon of light shining high in the west after sunset. This is Venus, the closest planet to the Earth and the brightest object in the sky next to the Moon and the Sun. Its brilliancy and proximity to the horizon as the evening progresses makes it easy to mistake it for an aeroplane coming in to land, sometimes it is even reported as a UFO.

Venus, along with Mercury, are the only planets in our solar system interior to the Earth’s orbit. For this reason, they always appear close to the Sun in the sky and can generally only be viewed in the west after sunset and in the east before sunrise. They also display phases similar to the Moon’s.

Through the telescope, Venus shows a bland, featureless appearance. What one sees is actually the top layer in a perrenial, unbroken envelope of clouds that encircles the planet. Astronomers wondered for centuries what the surface might be like, until spacecraft traveled to Venus and told us of extremely inhospitable conditions: temperatures of 500 C, hot enough to melt lead; crushing pressures, similar to the those at the bottom of the Earth’s oceans; and a corrosive, sulfuric acid rain (see recent post by Helen McLoughlin here). Though many countries and agencies have been sending probes to Venus since the 1960s, to date none have survived in this hellish environment for longer than the average length of a major feature film!

Though its surface is all but inaccessible, much progress has been – and is being – made in studying the thick atmosphere of Venus. Intense scrutiny by Earth-based telescopes and orbiting probes has shown that the clouds are transparent to wavelengths or “colours” of light invisible to the human eye, but detectable with electronic sensors, such as those found in commercial digital cameras. This has allowed scientists to peel away Venus’s cloud layers, much like an onion, and study how they change with height, across the planet and over time. For instance, infrared light emanating from the hot lower atmosphere all the way down to the surface is particularly suited to showing features on the lower cloud decks and track atmospheric circulation on the nightside.

The dayside part of Venus (left image) appears bland in visible light. Infrared light from the hot nightside (right image) shows off clouds and atmospheric circulation not visible to the eye.

These atmospheric “windows”, as they are called, are the primary means of studying Venus’ atmosphere. Images taken in ultraviolet light highlight areas of contrast in the uppermost layers, some 40 to 50 miles up and help trace out wind patters at those atitudes. The nature of the darkening agent or  “ultraviolet absorber” as it is known, is only one of the many unsolved mysteries associated with our planetary neighbour.

Close scrutiny by orbiting spacecraft, such as the Japanese Akatsuki probe that has been observing Venus since 2016, is now complemented by the work of amateur astronomers around the world.  These dedicated “citizen scientists’’ use off-the-shelf equipment and image processing software to produce high quality images rivalling spacecraft views of the planet.

The different colours of Venus. Left: View from Earth recorded by Mr Manos Kardasis from Athens, Greece, on 10 March 2020 when the planet was 76 million miles away. Red represents infra-red light, while blue represents ultra-violet light. Right: View from the Japanese orbiter Akatsuki (“Hope”) obtained on 22 August 2017 from 40,000 miles away. The colours correspond to slightly different wavelengths of ultraviolet light.

However, you don’t have to be an astromomer to enjoy the stark spectacle of Venus, plainly visible in the west right after sunset and outshining every other star in the sky. About a week from now, on the 26th of April, it will lie near a crescent Moon and help compose a nice photographic target. Then, a month later, it is joined by Mercury as that planet emerges from the Sun’s glare. On the 21st and 22nd of May the two planets are only a degree or so apart and the much brighter Venus serves as a useful signpost to spot the faint star of the innermost planet above a clear NW horizon.

Those in possession of a pair of binoculars and a tripod may be interested to try and make out the “horns’’ of Venus’s tiny crescent, only 1/60th of a degree apart at the time. A pair of 10×50 binoculars will magnify the apparent distance between the horns to 1/6th of a degree, which compares favourably with the resolution of the human eye, about 1/20th or 1/25th of a degree. Another chance to try this experiment will come at Venus’s next evening apparition in the summer of 2021 but the planet will then lie closer to the horizon and be more difficult to observe.


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