12Jul2012

Why Planet Venus Is So Bright in Mid-July 2012.

Early pre-dawn skies are just magnificent mid July

Venus shines at it brightest as the morning star for about a week or so during the middle part of July 2012. Astronomers call this a greatest brilliancy for Venus.

This brilliant planet shines close to another dazzling planet, Jupiter, on these July 2012 mornings, and it’ll easy be to tell which world is Venus and which is Jupiter. Venus is by far the brighter of the two. Venus ranks as the third-brightest celestial object in all the heavens and Jupiter the fourth-brightest, respectively, after the sun and moon.

July 12 marks a special day on the calendar for earthly observations of Venus. It’s the day on which Venus reaches what astronomers call its greatest illuminated extent in Earth’s sky. This event marks the moment when the visible fraction of Venus’ day side – the part we on Earth can see – covers the greatest area of our sky. It’s the reason Venus is so bright now.

What does it mean? Only that Venus is exceedingly bright now. It’s dazzling in our predawn and dawn sky, near Jupiter. If you look, you’ll find the planet looming low in the east. It’s almost eerily brilliant before sunrise.

Diagram of the phases of Venus in 2004. In that year, Venus reached its greatest brilliancy on May 3. So the phases corresponding to the mid-July, 2012 phase of Venus are those around 1/5/04 and 7/5/04 in this diagram. Copyright Statis Kalyvas – VT-2004 programme – via Wikimedia Commons.

Remember, Venus orbits one step closer to the sun than we do. This means two things. First, the disk of Venus (revealed by telescopes) is not always the same size in our sky. When Venus is far across the solar system from us – on the far side of its orbit – its disk looks relatively small from Earth. But around the time Venus passes between us and the sun – as it did on June 6, 2012 – the disk of Venus is relatively large in our sky. (Diagram above courtesy of Wikipedia. Phases of Venus copyright by Statis Kalyvas – VT-2004 programme.)

And at such times – when the disk is large, because Venus is nearby – we see Venus show phases, like a little crescent moon. Imagine if we could see the whole day side when Venus was closest – wow! Venus would be bright indeed. But you can’t see that. Instead, when Venus passes between us and the sun, its day side is facing away from us. That’s why Venus is always brightest when the planet is in a crescent phase – about five weeks before and after its passage between us and the sun – when the size of the disk in our sky is still large, but some significant fraction of the day side is showing.

Diagram of the phases of Venus

Unlike the moon, Venus wanes from full to new in the evening sky and waxes from new to full in the morning sky

And there you have it. Greatest illuminated extent. It’s the combination of big disk size for Venus and fraction of Venus’ day side visible to us. Although Venus’ disk will be less than 27% illuminated by sunlight tomorrow morning, Venus’ illuminated portion will cover more square area on the sky’s dome than at a fuller or thinner phase.

After tomorrow, Venus’ phase will actually increase – like a waxing moon. But the faster, smaller orbit of Venus will carry it farther ahead of Earth. The distance between our two worlds will be increasing. From Earth, we’ll see the overall disk of Venus shrink as its distance from us increases. Venus will dim somewhat as the morning “star,” but this dazzling world – as always – will continue to shine as the third-most-brilliant heavenly body, after the sun and moon.

If you’re up before the sun, that other brilliant star-like point of light near Venus is the dazzling planet Jupiter. Although fainter than Venus, Jupiter is the fourth-brightest celestial body to light up the heavens, after the sun, moon and Venus. Enjoy the spectacular morning drama, as Venus reaches its greatest illuminated extent on July 12, 2012 – shining at its brightest in mid-July 2012 in the east before sunup! Source: EarthSky