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B When are we "trailing" the Sun in its galactic revolution?

  1. Nov 12, 2016 #1
    Hi! I guess this question must be easy, but it's driving me crazy: in what time of the year does the Earth "trails" the Sun in its current galactic movement towards Vega? And, could you please confirm that during this period Vega is not visible because it's always facing the "day side" of the Earth, and thus "hidden" by the Sun?

    Thank you in advance!
  2. jcsd
  3. Nov 12, 2016 #2


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    This happens from October to April, however...

    that doesn't happen. Providing you're on the higher-mid latitudes of the northern hemisphere, you should be able to see Vega year-round.

    This is because the plane of the ecliptic does not coincide with the galactic plane, nor with the direction towards Vega (which is about 20° above the galactic plane). I.e. the solar system does not orbit the galaxy edge-on, but is inclined approx 60° w/r to the galactic plane, and some 30° 'sideways' w/r to the prograde direction of its galactic orbit. In yet another words, we're much more head-on than edge-on, so the Sun doesn't really get in the way.

    If you need visualisation, download a planetarium software (e.g. Celestia), go to options and turn on the following: galactic grid, ecliptic grid, orbits, constellations and star names. Technically, all you need is the two grids, but it might be hard to see what you're looking at. Zoom out so that you can see where the plane of the orbits is, and move the view around so that it points towards Vega.
    Should look like this:

    If you advance time rate enough, you'll see the Earth moving in its orbit. It's 'trailing' when it's on the left-hand half of the orbit as seen in this picture.
    Last edited: Nov 12, 2016
  4. Nov 12, 2016 #3
    Thank you very much, Bandersnatch! That's exactly what I needed! :smile:
  5. Nov 12, 2016 #4


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    tho there will be a period of months each year that it will be in the daytime sky, not the nite time sky, as will all other celestial objects
  6. Nov 12, 2016 #5
    That's what I meant! I am very interested in this. Please, could you clarify for me if Vega (or the stars around) can be seen during those Northern winter / Southern summer months from any observatory in the Earth (or its nearby orbital observatories) at any time of the day, or is its brightness totally "killed" by the daytime sunlight for the duration of this period (or a part of it)...?

    (I understand that the orbital observatories are not hampered by a bright day sky, but still, I'm not sure about how good they are resolving things around a bright Sun...)

    PS. Northermost optical Observatory on the Earth's surface seems to be Skibotn, Norway (Lat: 69º23'30", well above the Arctic Circle, but I'm not sure if they still have some diurnal "twilight" there...)
    Last edited: Nov 12, 2016
  7. Nov 13, 2016 #6


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    That's not how this works. Whether you'll be able to see Vega or not depends on what lattitude you're observing from. If you're say, at the South Pole, you won't be seeing Vega at all, ever. If you're on the other hand e.g. in Europe, you'll be able to see it all year round. You might just need to make the observations at different times of night (i.e. morning vs evening).

    @davenn that's not true for e.g. circumpolar stars - you can observe them at night all year round from the northern hemisphere. Vega is sufficiently close to the celestial pole that it's always visible at northern latitudes. Above 50-55N it never dips below the horizon, so it's just a matter of waiting for the sun to set.

    (and of course taking into account the fact, if you're above the polar circle, there is a period of time when the sun doesn't set - but there's a range of latitudes that should allow viable viewing at least for some time every night)
    Last edited: Nov 13, 2016
  8. Nov 13, 2016 #7
  9. Nov 14, 2016 #8
    A few years ago we did a midsummer nights limiting magnitude test on a Swedish astronomy forum. I was the northern most data point at 62.4 deg N (middle of Sweden) and Vega, close to zenit, was one of two stars I could spot. You could probably spot it a little further north on midsummer but the limiting magnitude in the summer drops off quickly close to the polar circle.
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