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Why is Venus the Brightest?

March 11, 2009 on 9:59 am | In Astronomy, Solar System |

Sometimes, when you look up in either the early, pre-dawn morning or early, post-sunset evening sky, there’s one point of light that outshines all the others. It’s usually relatively close to where the Sun was, and unlike most points of light you see in the night sky, this one doesn’t twinkle. I’m talking, of course, about the planet Venus:

Yes, it isn’t nearly as bright as the Moon, but it’s certainly much brighter than everything else you can see. Well, it was only a matter of time before someone wrote in and asked why. Reader Dan asks:

Can you explain why Venus is so bright in the sky right now? I don’t think I have ever seen it so bright in my life.

Venus, as we’ve talked about before, is covered in a very thick layer of greenhouse gases. This makes its surface extremely reflective, so that about 70% of the sunlight that comes in to Venus gets reflected as visible light.

But Venus is also extremely close to Earth. In fact, Venus gets within about 40 million km of Earth, which is less than 1/3 of the distance to the Sun. At its farthest, Venus is about 250 million km from Earth. Why? Because Venus and Earth both orbit the Sun; sometimes they’re on the same side as one another, and sometimes they’re on opposite sides. When they’re both on the same side, Venus has its closest approach to Earth:

But I’m going to force you to think about the geometry of this a little bit. One side of Venus faces the Sun, and gets illuminated; the other one is dark. Imagine that the Earth is far away from Venus. What will the planet Venus then look like from Earth? It will be mostly “full”, and the closer to being in direct opposition to Earth, the more “full” Venus will appear. But the more “full” it appears, the further away it is, and so it should also look smaller. Take a look at these shots of Venus, through the same telescope, when it’s far away from Earth:

When it’s more full, it’s also smaller. But when less of it is illuminated, it’s closer to us. Take a look at what Venus looks like when it’s closest to us:

So when Venus is close to us, it looks like a crescent, but it looks like a very large crescent, and when it’s far from us, it looks like a disc, but a much smaller disc. Which of these two things is more important for the brightness of Venus?

Well, we can find out. You see, Dan is right; Venus is nearly at its brightest right now. Have a pair of binoculars? Well, about 11 days ago, somebody did, and photographed Venus and the Moon together in the sky. Here’s what they saw:

Venus is at its brightest when it’s a crescent! It turns out that being closer to us means everything. Astronomers use magnitudes to measure brightness, and the smaller your number is, the brighter you are. This is useful for stars, where very bright stars are typically 0 or 1 (the very brightest, Sirius, is -1.5), and the dimmest ones visible with the naked eye are 5 or 6.

What about things that aren’t stars, though? Not surprisingly, the Sun is the brightest, at -26.7, and the Full Moon is second, at -12.6. But Venus is third! When Venus is a crescent, it’s -4.6, and when it’s full (but far away), it’s -3.8, which is interesting! Why? Because when it’s full, you can’t see it during the day, even under optimal conditions. But right now, when it’s a crescent, you can! Check out this photograph by John Harper of Venus during the day:

It’s a crescent, just like we said it should be! So that is why Venus is the brightest thing in the sky, and that’s why it’s brighter now than it usually is. Get out your binoculars and have a look! And if you do it tonight, look on the other side of you too, because the Moon is full and it rises at 8:30 PM! Enjoy!


27 Comments »

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  1. I’ve really been enjoying having it in the sky lately. How long will it remain bright and easily visible? (out at night instead of during the day, etc) Orion’s going away for the summer, and I like having something up there I can identify. :P

    Comment by benhead — March 11, 2009 #

  2. thanks for answering my question! I can’t wait to show my kids this. Keep up the good work, it was a very informative article.

    Comment by Dan — March 11, 2009 #

  3. Great article, Ethan!

    Comment by Richard Helmich — March 11, 2009 #

  4. […] With a Bang! gives us the lowdown on why Venus seems really really bright sometimes in ‘Why is Venus the Brightest?‘ It’s quite informative, and I think Starts With a Bang! should visit Venus more […]

    Pingback by The 94th Carnival of Space - Out of the Cradle — March 14, 2009 #

  5. wow this is really interesting i really would like to learn more

    Comment by Brianna — March 30, 2009 #

  6. wow this is really interesting i really would like to learn more =)

    Comment by Brianna — March 30, 2009 #

  7. wow this helped me so much in my homework thnks

    Comment by jas jas beybeh — April 21, 2009 #

  8. why do shooting stars have their different colors when they shoot?

    Comment by Ashley — June 19, 2009 #

  9. this is really coooooooooooooooooollll … so fetch :)

    Comment by mitchell deruelle — July 31, 2009 #

  10. Hi, Ethan:

    Thanks for the info and corresponding pictures about Venus. Am using the pictures in my classes.

    Comment by Linda Vilma A. Ole — September 14, 2009 #

  11. what!!!!!!!!!!!!!!!!!!! i cannot see planet venus

    Comment by chris — November 3, 2009 #

  12. hi … i just have a small doubt..

    i have learned that greenhouse gases trap heat waves etc in atmosphere andthey cant escape, which causes global warming.

    but here it is said that the greenhouse gases cause more reflection.
    How is this possible??

    pls mail me the answer at akavcet@gmail.com if anyone knows..

    Comment by arunkumar — January 26, 2010 #

  13. Fascinating as the planet Venus is, thru a telescope its rather bland.

    Comment by James — July 3, 2010 #

  14. what is the use ofvenus

    Comment by prammanantham — July 31, 2010 #

  15. Hi there mate i have a mate who is adiment that this bright “star” is infact the planet nebiru can you pls help out with this, ive always been told that the brightest star in the sky is venus.

    Comment by tony — August 13, 2010 #

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    Pingback by Cooler By the Lake — June 14, 2011 #

  17. Thanks so much for giving everyone such a nice opportunity to discover important secrets from this website.

    Comment by Callie Silloway — September 6, 2011 #

  18. its firs date to this blog, its nice ! keep work…..

    Comment by JOCURI SLOT — November 13, 2011 #

  19. try as I may getting my head around venus in 2012 being so bright brighter than I have ever seen it in all my years does seem to worry me some what (probably listening to all these forcasts about 23/12/2012 lol) but even when I was a small boy and the sky was crystal clear this time of year venus was not much bigger than a star is this due to the earths axis on its rotation around the sun now that makes it look so much bigger? ps the moon is not as bright as venus!

    Comment by george — February 29, 2012 #

  20. Thanks for the insight… this will cap the conspirationists that impending doom is upon us… watch out its Nibiru
    we have a part of the universe in us and everything is connected as a whole through electricity…Mmmm, to be confirmed…

    Comment by andr√© — March 21, 2012 #

  21. I am a scientist. I turned 60 last year. I took astronomy in college. Throughout my life I have seen at least five comets, several conjunctions and other phenomena, and I have observed Venus many many many times. HOWEVER, in all my life I have never observed Venus even 1/10th as huge, incredibly, indescribably brilliant, or as high in the sky, or as late at night, as it has been for the past two months (Mar & Apr 2012). Even its temporary companion Jupiter, which as of Apr 20th had moved westward to the horizon, far away from Venus, and which is now only briefly visible after sunset, was vastly larger and brighter this year than ever before! No one has been able to explain why Venus has dominated the sky this year, but everyone has been talking about it. It’s so stunning and incredibly awesome! I have spent hours each week being dazzled by it. Is it Man’s destruction of our atmosphere? Has Venus, Jupiter, or Earth changed orbit? What has changed? In the absence of a detailed explanation, I JUST CANT BELIEVE IT’S VENUS ???

    Comment by Venus Nut — April 24, 2012 #

  22. Excеllent post. I dеfіnitеly aррreciate
    thіs ѕitе. Stick wіth it!

    Comment by diigo.com — November 5, 2012 #

  23. brilliant, or as high in the sky, or as late at night, as it has been for the past two months (Mar & Apr 2012). Even its temporary companion Jupiter, which as of Apr 20th h

    Comment by sohbet — February 6, 2013 #

  24. I do not write a leave a response, but I looked at a few of the responses
    on this page Why is Venus the Brightest? | Starts With A Bang!

    . I do have 2 questions for you if you tend not to mind.
    Is it just me or do some of the remarks come across like they are written by
    brain dead folks? :-P And, if you are posting at additional
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    Comment by proba — April 27, 2013 #

  25. Venus is so bright it is as big as earth ,it comes relatively close from time to time and its atmosphere is all one thick cloud that reflects sun to us like someone flashing a mirror , Venus goes around the sun faster than earth it comes from behind slowly comes along side between us and the sun then pulls slowly ahead of us we see the bright crescent close in and closer to a disk as it pulls away . I think when it shows somewhere between a quarter and full disk it is the brightest but I may be incorrect on that I am but a dillitant .
    She sure is brilliant as the evening star Prettiest I have seen in years I am in Vancouver at the 49 N and was high and bright and just bit west of south but not much I had a high horizon due to the slope of a hill but it was 15 above tops of the buildings. The next night i was in the same place and watched it appear as the sets and it shows up in a sky that is still light because the sun sets behind the same slope before the sun actually sets on the true horizon so Venus shows up while the sky is still blue and it stands out in 3d where the sky darkens it becomes 2d I almost looks as if it is in our atmosphere so i understand why people might report if as a UFO And to someone with bad eyesight could see the bright crescent as saucer shaped if the angle of the curve was right

    Comment by abtony peart — November 27, 2013 #

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  27. is stupid

    Comment by bob jr — February 20, 2014 #

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