The Planet Venus


The Planet Venus Compared to the Earth

Comparative scale views of the Earth, the moon and the planet Venus.

The planet Venus is sometimes referred to as Earth's sister planet due to it being similar in planetary type, size and mass to the Earth. The Earth is a just a little larger with a mean radius of about 3955 miles (6365 km) while the radius of Venus is 3761 miles (6052 km) or 95.09 percent of the 'size' of the Earth.

Since it is closer to the Sun than is the Earth, Venus never gets farther than about 48 degrees from the sun, so it generally rises not too long before or sets not too long after the sun.

Viewed by the naked eye, Venus appears brighter than any star, at a visual magnitude of up to -4.5, its brilliance outshining all but the Sun and moon, sometimes even casting a shadow.  It is often seen appearing as the brightest 'star' in the early morning or early evening sky.  Under excellent viewing conditions, Venus is so bright that it can sometimes be observed even during the mid-afternoon, in full daylight if one knows where to look.

When not recognized, Venus is often reported as a UFO, since it can put on quite a light-show, especially when close to the horizon in areas where the weather is warm and humid.  When seen in the sky during the Christmas season, due to its outstanding brilliance, Venus is often considered reminiscent of the Christmas star (the star of Bethlehem).

A distant bright light source could be radiating at a steady brightness level, but from miles away, heat ripples in the atmosphere could make that steady light source appear like a distant flame or candle fluttering in the breeze.  The same thing happens with the stars and planets in the sky.

Planets generally show less twinkle than stars, nevertheless, all the planets can twinkle like stars, even the brilliant Venus.  It just depends on the atmospheric viewing conditions.  The twinkling effect is primarily due to heat ripples in the air, varying pressure gradients, random atmospheric turbulence across the many hundreds of miles of atmosphere between the observer and the object of observation.  Generally, the warmer the air and/or the closer to the horizon, the greater the twinkling (heat ripple) effect.

When viewing the stars and distant planets from outside the atmosphere, such as from a spacecraft or from the surface of the moon, neither stars nor planets would appear to twinkle, simply because there is no atmosphere to cause it.

Simulated telescopic views of Venus

A telescopic view of Venus from the Earth reveals a very bright, whitish disk that over the year, slowly goes through all the phases similar to the moon, from new to full.  Its extraordinary brightness comes from its light clouds which cover the entire surface, brightly reflecting the sun.  It is unlikely that any faint streaks in the clouds of Venus would be visible from Earth through an ordinary telescope.  Depending on its position in the sky and the local area viewing conditions, the appearance of Venus may significantly vary in sharpness and clarity from one observation to the next, sometimes being sharp and clear and at other times, blurry.  This is true of any planet observed through a terrestrial telescope.

The Phases of Venus

Since the orbit of Venus is inside the orbit of the Earth, like the moon, it goes through a similar cycle of phases from new to full.  However, the brightness of Venus with respect to its apparent phase varies in a manner entirely different from the moon.

As Venus approaches its full phase, it gets fainter and fainter, while the brightness of the moon increases as it approaches full moon.  As Venus approaches a crescent phase, it reaches its greatest apparent brightness, while the brightness of the moon decreases as it approaches a crescent phase.  This is due to the varying apparent size of the planetary disc as it cycles through the phases.  When Venus is in a crescent phase, it is much closer to Earth than it is when it is at full phase.

The farthest-to-nearest distance ratio is about 6:1, which means that when Venus is at its greatest distance from Earth, it is about 6 times farther away than when at its closest approach.  This also means that the apparent angular diameter of Venus is about 6 times greater when at its closest approach than it is when at its greatest distance from Earth.  This is a significant difference.

Venus becomes a very thin crescent as it approaches its closest approach or shortly after passing this point between the Earth and sun, reaching its maximum angular diameter when in its new phase, at which time it is invisible except on those rare occasions when it transits (passes across) the face of the sun as it did on June 8, 2004 (UT) and will again on June 6, 2012 (UT). 

When Venus is in its full phase, then it is on the far side of the sun, at its greatest distance from Earth at the time and at its minimum apparent angular diameter of about 1/6 its diameter at closest approach.

... work in progress ...

© Jay Tanner - 2012