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Venus is the brightest planet in our sky, a brilliant white jewel that can cast a faint shadow on a dark country lane. Yet point even a modest telescope at it and you may be surprised: Venus is not a featureless disc. It shows phases, just like the Moon. One month it is a fat gibbous shape, the next a slender crescent. The reason is not a shadow falling across it. It is geometry: Venus orbits closer to the Sun than Earth does, so its sunlit half turns towards or away from us as the two planets travel around the Sun.
Astronomers call Venus an inferior planet because its orbit lies inside Earth's orbit. Like every planet, Venus is lit by the Sun from one side only. Half is always in daylight, half in night. From Earth we see that illuminated half from different angles as Venus moves around the Sun. When Venus is on the far side of the Sun, we see its fully lit face, so it looks full or gibbous. When it is roughly between Earth and the Sun, its sunlit side faces away, so it appears as a thin crescent or vanishes in the Sun's glare. The phase depends on the angle between the Sun, Venus and Earth—the same principle that gives the Moon its phases.
Venus takes about 225 Earth days to orbit the Sun, but its phase cycle as seen from Earth lasts roughly 584 days because Earth is also moving. At inferior conjunction, Venus passes between Earth and the Sun. It is invisible, lost in solar glare, and at its closest. A few weeks later it emerges as a morning star, low in the east before sunrise. Through a telescope it is a large, thin crescent. As it climbs higher, the crescent thickens. At greatest western elongation it is farthest from the Sun in our morning sky and half-lit, like a first-quarter Moon. It then swings around the far side of the Sun, becoming gibbous and smaller because it is farther away. At superior conjunction it is fully lit but hidden. It reappears as an evening star, gibbous at first, then shrinking to a crescent as it returns to inferior conjunction. The most striking crescent phases occur when Venus is closest to Earth; the full or gibbous phases occur when it is farthest. That is why Venus looks brightest not when full, but when it is a fairly thin crescent: it is then much closer, and its lit portion is nearer to us.
Your eyes alone will not show the phases. Venus remains a point of light, however bright. But you do not need expensive equipment. A pair of 10x50 binoculars, steady against a wall or on a tripod, will show a clear crescent when Venus is near inferior conjunction. A small telescope of 60mm to 100mm aperture, used at 30x to 100x, will reveal the phase throughout most of the apparition. The main problem is glare. Observing in twilight, before the sky is fully dark, helps, as does higher magnification. A variable polarising filter or a simple aperture mask can reduce glare, but try without first. Never point a telescope or binoculars at or near the Sun: even a glimpse can cause permanent eye damage. Wait until the Sun has fully set for an evening star, or observe before sunrise for a morning star. Choose a spot with a clear eastern or western horizon, because Venus never climbs high in UK skies. Keep a sketchbook: draw the phase and note the date.
In 1610, Galileo turned an early telescope on Venus and saw something that changed astronomy. He found that Venus goes through a full set of phases, from crescent to gibbous, and that its apparent size changes in step with those phases. In the older Earth-centred model, Venus was thought to circle Earth while staying near the Sun. That model could allow some phases, but it struggled to explain the complete range and the way Venus grows larger when it is a crescent. A Sun-centred model, with Venus orbiting the Sun inside Earth's orbit, predicts exactly what Galileo saw. Venus is a crescent when it is between Earth and the Sun, and gibbous when it is on the far side. The phases of Venus became powerful evidence that the planets orbit the Sun, not Earth.
From the UK, Venus is never high in a fully dark sky. Here are some practical pointers for your first phase watch.
Venus rewards patience. It is easy to see, but its phases turn a bright point of light into a moving, changing world. On a crisp UK twilight, with a small telescope and a clear western horizon, you can watch the same cycle that puzzled astronomers for centuries, and see it with your own eyes.
Aster Report shares practical, down-to-earth guidance on amateur astronomy and stargazing for beginners in the uk for readers across the UK.
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