Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I Mercury can be seen at night?

  1. Oct 21, 2016 #1
    Hello,
    I have been doing extensive research into planetary positions and distances from the sun. When viewing the sky for planets it has been noticed that Mercury is in the night sky. Mercury is at 36million miles from the Sun. Earth is 93million miles from the Sun. How can the planet Mercury be seen at night when it is orbiting the Sun much tighter to it then Earth? Based on drawing diagrams to replicate what I can see I can not find an angle that shows Mercury visible at night. What am I doing wrong?
     
  2. jcsd
  3. Oct 21, 2016 #2

    1oldman2

    User Avatar
    Gold Member

  4. Oct 21, 2016 #3
    Thank you for your response. ^-^ Naturally I did use google in order to search for an answer that matches what is seen in real time with a telescope. The article provided was already considered and did not explain why I did not see Mercury following around the sun. It appeared that Mercury was in an impossible orbit. Being that the planet should only been seen in the day as even the article you sent is also the assumption I was under how can it be seen at night by myself and others once the Sun has moved behind the Earth?
     
  5. Oct 21, 2016 #4
    Please if your browser allows use this picture as reference of positions of earth in relation to Mercury. How can the night side of the Earth shown in the pic ever possibly see Mercury at night up in the sky? http://www.universetoday.com/wp-content/uploads/2013/05/Orrery-view-May27-580x430.jpg
     
  6. Oct 21, 2016 #5
    Mercury can only be seen from Earth at times where it has it's largest angular separation from the Sun.
    In practice this means it can only be seen low in the night sky.
    Either soon after the Sun sets, (and it too then 'sets'), or other times it can be seen 'rising' just ahead of Sunrise.
    It's still there of course, close to the Sun, during daytime, just that we can't see it.
     
    Last edited: Oct 21, 2016
  7. Oct 21, 2016 #6

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    A. That picture is not to scale.
    B. Venus is visible, why not Mercury?
    C. In that picture Mercury is behind the sun. Suppose it was slightly more counterclockwise in its orbit.
     
  8. Oct 21, 2016 #7

    davenn

    User Avatar
    Science Advisor
    Gold Member

    Mercury is ONLY seen in the early evening for a short time, usually not much more than an hour at the best times ... when at greatest angular separation from the sun, after sunset ( at times it isn't behind or in front of the sun)

    It will NEVER be seen higher in the night sky

    the same goes for Venus, except it gets a bit higher up into the sky from the horizon and is visible for a longer time


    Dave
     
  9. Oct 21, 2016 #8
    Yes I do agree with this when I have observed Mercury personally. It does not sit in the sky all night long. Only for a awhile just after dark and at times before the Sun rises. What we discern orbits by is based on the shadows of the planet facing away from the Sun. The lit side is an indication of which side of the Sun and what direction it would be moving during day to day comparison. What is not observed personally at any time viewing Mercury over long periods of time is a shadow side. When I will reference what should be seen on current planetary charts, that display which direction the shadow should be on, I can still not see this shadow. It's as if Mercury is always fully circular with the light being fully facing us just like the Moon appears during a full Moon phase. Based on the face being entirely lit it appears to me to be in an impossible orbit location. What can I do to replicate what I am supposed to be observing?

    I have however seen photos of what a transit may look like and can agree that it does appear to be dark on the side facing the Earth. Up until that time tho its been described to be quite visible with correct filters on a telescope as a point of light until this encounter with the Sun. I have not personally seen this so I am only going on information presented out publicly. What my current questions stem from are only based on personal observations.
     
  10. Oct 21, 2016 #9

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    You can't see the unlit side because it's unlit. You can, however, see phases, like you do for the moon and Venus. If you Google "phases of mercury" you will see many pictures.
     
  11. Oct 22, 2016 #10

    davenn

    User Avatar
    Science Advisor
    Gold Member

    during transit, of course Mercury and also Venus for that matter, are going to look dark, you are seeing them in silhouette against the very bright disk of the sun

    here's a pic of a transit of Venus I did 6 June 2016, Mercury will look the same just smaller .... I have photo'ed a Mercury transit but before the days of digital cameras and don't have a scanned copy

    upload_2016-10-22_19-31-28.png


    cheers
    Dave
     
  12. Oct 22, 2016 #11
    Is this a real unedited photo of the Sun? This is not what I see when I view the Sun. What brand of filters and program do you use?
     
  13. Oct 22, 2016 #12
    I'm curious as to the exact angle and calculations for the distance of visibility of Mercury that can explain how it is possible for Mercury to be seen for a small duration of night just after complete sunset and for a small duration before light being visible for the sunrise. Does anyone here have this formula that I can look over and apply to my observations to better make sense of all this?
     
  14. Oct 22, 2016 #13

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

  15. Oct 22, 2016 #14

    tony873004

    User Avatar
    Science Advisor
    Gold Member

    The size of Earth is exaggerated here. This image shows the Sun, orbit of Mercury, orbit of Venus, and the orbit of Earth in June 2015, when Mercury was near maximum elongation. It appeared as a morning star shortly before sunrise.


    mercurySun.GIF
     
  16. Oct 22, 2016 #15

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    It would be good to get this right before trying to go any further. This statement implies that Mercury is at the same orbit distance as Earth. It is much, much closer - between about 0.3AU and 0.45AU. This link has more details about the situation. The greatest angular separation between Mercury and the Sun is about 28° but then only half the illuminated part of the planet is visible (A 'half moon situation) . When it's further round in its orbit, you will see more of its illuminated bit but it will appear closer to the Sun.
    The sky will not look absolutely dark in that direction, even when the Sun is well below the horizon and Mercury will be close to the horizon so conditions need to be just right to see it. If you want to be there when it happens, get hold of Stellarium (free) and it will tell you which nights that condition is satisfied.
    You may find that you can answer your own original question (the geometry) by drawing the planetary orbits and the Sun to scale, using the distances in that link. You can then check it against what Stellarium tells you (if you in put the right time and day). Stellarium is fairly intuitive but it does require investing an hour or two in just getting to know it. It's very pretty so it is not hard work!
    How could it? It's only just behind the Sun in its journey round the back of the Earth (Earth reference frame, of course)
     
  17. Oct 22, 2016 #16

    davenn

    User Avatar
    Science Advisor
    Gold Member

    yes a real unedited pic other than a tiny bit of sharpening. There is a solar filter across the front of the lens, all it does is cut out
    99.9% of the light intensity

    upload_2016-10-23_9-15-46.png

    Dave
     
  18. Oct 22, 2016 #17

    davenn

    User Avatar
    Science Advisor
    Gold Member

    ahhh @Sophie, you better reconsider that comment :wink:
    the number is correct, it varies from around 28 to 43 million miles

    earth is much further, as the OP said, at 93 million miles

    Dave
     
  19. Oct 22, 2016 #18

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Ye gods!!! I just read those numbers wrong. What a plonker. Sorry chaps (Porsha particularly)
    The max angle between Sun and Mercury is pretty small, though and so Mercury is nearly always outshone by the Sun.
     
  20. Oct 22, 2016 #19

    davenn

    User Avatar
    Science Advisor
    Gold Member

    happens to the best of us :wink::smile:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted