The sky — astronomy

Venus: The Evening and Morning Star

The very bright “star” you see after sunset or before sunrise is the planet Venus. It shines so brightly because its thick clouds reflect a large share of sunlight and because it comes relatively close to both the Sun and Earth. The Evening Star and the Morning Star are simply the same planet, seen at different times of day.

If you have noticed an unusually bright light low in the sky shortly after sunset or before sunrise, you were most likely looking at Venus. It is the second planet from the Sun, and after the Sun and the Moon it is usually the brightest light in our sky. Here is what science tells us about it.

Why Venus is so bright

Venus makes no light of its own. It reflects sunlight, and three things make it so bright.

First, its clouds. The planet is wrapped in a dense layer of clouds containing sulfuric acid, which reflects roughly 70% of the sunlight that reaches it. Earth reflects about 30%. Scientists call this reflectivity albedo.

Second, it orbits closer to the Sun than we do, so it receives much more sunlight than Earth.

Third, it can come relatively close to us. At its best, Venus reaches an apparent magnitude of about −4.6. In this scale, more negative means brighter, so Venus far outshines Sirius, the brightest star in the night sky (Sirius).

Planets also tend to shine with a steadier light than stars, which twinkle. That can help you tell them apart.

Evening Star and Morning Star: one planet

Venus orbits inside Earth's orbit, so from our viewpoint it never strays far from the Sun. Its greatest angular distance from the Sun, called its greatest elongation, is about 47°. That is why you only see it in twilight: low in the west after sunset, or low in the east before sunrise.

When it appears in the evening sky, people call it the Evening Star. When it appears before dawn, it is the Morning Star. It is the same world, moving around the Sun and changing its position relative to us. The pattern of Earth, Venus and Sun lines up again roughly every 584 days. Ancient cultures described Venus in different ways, and in the Greek tradition its two appearances had separate names before it was understood that they were a single object.

To find it yourself, see How to watch the sky. Venus sometimes appears near the Moon or other planets; see Planet parade for such gatherings.

A thick atmosphere and a runaway greenhouse

Venus is only slightly smaller than Earth, but its conditions are very different. Its atmosphere is about 96% carbon dioxide. Pressure at the surface is roughly 90 times that on Earth, comparable to being about a kilometer deep in our oceans.

This dense carbon dioxide produces a powerful greenhouse effect: sunlight gets in, but heat has great difficulty getting out. Surface temperatures are about 465 °C, hot enough to melt lead. Venus is hotter than Mercury (Mercury), even though Mercury is closer to the Sun. It is the hottest planet in the Solar System.

Because visible light cannot pass through the clouds, the surface has been studied with radar. NASA's Magellan spacecraft mapped most of it this way in the 1990s, and ESA's Venus Express studied the atmosphere and clouds. New missions are being prepared, including ESA's EnVision.

Venus spins backwards

Venus rotates very slowly, and in the opposite direction to most planets. Seen from above the Solar System, most planets spin counterclockwise, while Venus spins clockwise. One full rotation takes about 243 Earth days, which is longer than its year of about 225 Earth days.

As a result, if you could see the Sun through the clouds, it would rise in the west and set in the east. A full day-night cycle, from one sunrise to the next, lasts about 117 Earth days. Why Venus rotates this way is still debated, and scientists consider several possible explanations.

This is not the same as “retrograde motion” in our sky, an apparent effect where a planet seems to move backwards against the stars for a while. It is explained in Retrograde motion, and its astrological reading is covered in Venus retrograde.

The phases of Venus

Like the Moon, Venus goes through phases, from a thin crescent to an almost full disk. They appear because we see more or less of its sunlit side depending on where Venus is relative to the Sun and Earth. Lunar phases are described in Moon phases.

When Venus is close to us, it looks large but is a thin crescent. When it is on the far side of the Sun, it looks small but nearly full. You can see this with a small telescope, or even good binoculars (see Binoculars or telescope).

In 1610 Galileo Galilei observed these phases through his telescope. They do not fit a model in which everything circles Earth, and they became an important argument for the Sun-centered picture of the Solar System.

In the astrological tradition, Venus is associated with beauty, love and values. That is a cultural interpretation, not a scientific claim. You can read more in Venus in astrology.

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