0

I was looking in the StarTracker android application from Tehran, Iran; and this is the screenshot:

StarTracker

As you can see the Moon covers a small portion of the Sun. The green line above them is the astronomical horizon and the two objects are located at NW direction.

I don't know if it's accurate. But how do people on the opposite side of the Earth (if we draw an imaginary straight line twoards the Sun from my location) see the Moon and the Sun if we assume the location of the objects are accurate on the app?

Snack Exchange
  • 995
  • 7
  • 20
  • 5
    Please clarify what you are asking. There are several different things that you could be asking, such as:
    1. Someone on the opposite side of the Earth cannot see the Sun and Moon at the same time: the Earth is in the way! Are you asking how the position and size would look if you could see them through the Earth?
    2. Are you asking what would the separation be approximately 12 hours later when someone on the other side of the Earth could see them?
    3. Are you asking about where they would be in the sky? (NW? some other direction?)
    4. Are you asking for something else?
     – JohnHoltz Sep 14 '23 at 21:12
  • 3
    The diameter of the moon is way larger in comparison to the Sun's diameter than they ever appear. So the diagram is not accurate. Same for the diameter of Mercury. The app is likely making them larger just to make them easier to identify. – Greg Miller Sep 15 '23 at 03:01
  • 1
    from the relative size of mercury it seems clear that those are not to scale – njzk2 Sep 15 '23 at 20:41
  • 2
    @njzk2 No one says scales are accurate. If they wanted to set scales correct they had to depict Mercury with a little sand of salt which would be hard for us to notice. Or the Sun with a disc as large as a great house which wouldn't contain in monitor. – Snack Exchange Sep 15 '23 at 20:48
  • 3
    "As you can see the Moon covers a small portion of the Sun" - This is only true if the scales are accurate. "No one says scales are accurate" - You can't have it both ways. If you are assuming that the moon covers the sun, then you are implying the scale are accurate. If you're saying the scales are not necessarily accurate, then you have no way of knowing if the moon covers the sun or not. – Todd Wilcox Sep 16 '23 at 04:56

2 Answers2

9

At the time depicted, it is close to a astronomical new moon, and night-time in Tehran. So at that time you can't see the sun nor the moon in Tehran

For people on the other side of the world (i.e. somewhere in the South Pacific Ocean) it is day-time. And the sun and moon are above the horizon. The sun is visible but the moon is not visible at new moon, as it is lost in the glare of the sun.

Of course, twelve hours later the situation is reversed, and it would be day-time in Tehran and night-time on the other side of the world.

Note that while the positions of the sun and moon are correct, their sizes are not. In fact they are both about the size of Virgo's eye (in the drawing used in that app) And at that time, the moon does not cover any part of the sun.

Moreover for some reason that app is using a picture of the planet Mars to represent Mercury.

James K
  • 120,702
  • 5
  • 298
  • 423
  • 1
    Should be noted that 12 hours later the Moon will move significantly from its current position relative to the stars, about 6-7 degrees, more than 10 times of its diameter. The values here are imprecize because of eccentricity of the moon's orbit. – Heopps Sep 15 '23 at 08:10
  • Mars itself is represented by a very red disc that has a little groves on it. – Snack Exchange Sep 15 '23 at 08:52
4

If the sky map is accurate, it will by definition look like the map (otherwise the map isn't accurate). Your position on earth changes very little about the positions of celestial bodies relative to one another, it only changes which ones you're able to see without the ground getting in the way.

Since most celestial bodies are many millions or billions of miles away, moving a few thousand miles in any direction does not introduce parallax apparent to the naked eye. The moon is the only body close enough to have noticeable parallax, and even that is quite minor. About the only time it would matter is when the moon is within a degree of occluding something as in a total solar eclipse, which is only perfectly aligned from limited viewpoints on earth.

If you can see the sun and moon, they'll look like the map. If you can't see them, they are still in the same relative positions, they're just below the horizon.

Nuclear Hoagie
  • 2,154
  • 15
  • 14
  • Lunar parallax is large enough to be visible to the naked eye. See https://astronomy.stackexchange.com/q/43717/16685 & http://www.etwright.org/astro/moonpar.html – PM 2Ring Sep 14 '23 at 19:57
  • 1
    @PM2Ring In a side-by-side comparison, yes, but we're talking about parallax on the order of a degree - in a general qualitative sense, it looks basically the same. I certainly have never noticed that I could see slightly different parts of the moon's surface at moonrise and moonset due to parallax. I expect the only time you'd actually notice a striking difference is if there is something to occlude within a degree of the moon, as in the case of the total solar eclipse which I mentioned. – Nuclear Hoagie Sep 14 '23 at 20:17