Does a Geo-Synchronous Satellite Catch a Shadow?

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Discussion Overview

The discussion revolves around whether a geo-synchronous satellite can cast a shadow on the Earth when aligned with the Sun. It explores the implications of satellite size, distance, and the geometry of light from the Sun.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants question if a geo-synchronous satellite, when aligned with the Sun, can cast a shadow on Earth.
  • One participant suggests that the size of the satellite is too small to create a significant shadow, referencing the Moon's ability to cause eclipses only under certain conditions.
  • A mathematical explanation is provided, indicating that a satellite would need to be within approximately 229 meters to cast a proper umbral shadow, based on its size and the angle of sunlight.
  • It is noted that satellites typically orbit at altitudes of tens to hundreds of miles, making it unlikely for them to cast a shadow that reaches the ground.
  • A link to a visual representation of satellite transits across the Sun is shared, suggesting that while satellites can transit the Sun, they do not create shadows on Earth.

Areas of Agreement / Disagreement

Participants generally agree that the size and altitude of geo-synchronous satellites prevent them from casting noticeable shadows on Earth, but the discussion includes varying details about the geometry involved and the specific conditions required.

Contextual Notes

The discussion includes assumptions about satellite sizes and distances, as well as the geometry of light from the Sun, which may not be universally applicable to all satellites.

ucmassahana
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If a geo-synchoronise satelite(which has the same time period as that of the earth) comes in a straight line with the Sun. Does it catch the shadow on the earth? If not why?
 
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ucmassahana said:
If a geo-synchoronise satelite(which has the same time period as that of the earth) comes in a straight line with the Sun. Does it catch the shadow on the earth? If not why?
Do a search on google for umbra and penumbra
 
Basically, it is too small.

Even the Moon, doesn't always cause a full eclipse - sometimes it is just slightly too far away (It's orbit isn't a perfect circle), and some sunlight gets round the edges.

The Sun is about half a degree across - so the rays of light from each limb (side) as we look at it, are not parallel - they are converging (from our viewpoint) at an angle of about half a degree.

So if you do the maths:-
For a 2 metre wide satellite...
Right angle triangle - quarter of a degree angle - 1 metre 'opposite' side (half the width of the satellite)
Tangent of 0.25 degrees = one metre / adjacent side (distance behind satellite where light rays will converge

0.00436 = 1 / adj
adj = 1 / 0.00436
adj = 229.3 metres

So you'd need to be within 229 metres of the satellite to get a proper umbral shadow.

Obviously - different satellites will be different sizes to the one in my example - so some will throw a shadow quite a bit more than 229m.
However - satellites are AT LEAST tens of miles up, many are hundreds of miles above the surface - so even the largest ones won't throw a shadow anywhere near far enough for it to be seen on the ground.
 
Last edited:
Well, here's what it looks like with a telescope and a solar filter... http://www.badastronomy.com/bablog/2006/09/20/shuttle-and-iss-transit-the-sun/
 

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