While pulling weeds from the flowerbeds today, a shadow passed over me. I was looking groundwards, but from my peripheral vision I could tell it was a large shadow. When I looked up, however, not one second after the shadow passed, there was nothing but clear blue sky and a white-hot globe. No plane, no bird, nothing.
Is it possible it was a satellite? Could an orbiting satellite of relatively small size cast such a distinct shadow? I guess I would have figured something so comparatively small and distant wouldn’t be able to cast an actual shadow, as the light would just diffuse around it. Perhaps that was a poor assumption though, or I’m just going loony or maybe loosing my eyesight.
The sun is not a point source of light; the solar disc subtends a non-zero angle across one’s field of view. As a result, illumination of an object by the sun causes it to cast an umbra (a region of shadow where the view of the sun is completely obscured) and a penumbra (a region of shadow where the view of the sun is only partially obscured).
In the case of an artificial satellite with a dimension no bigger than a few yards orbiting at an altitude of no less than 150 miles (possibly much higher), the umbra down at the surface of the earth would be nonexistent, and the penumbra would be an area in which the view of the sun is only barely obscured by the satellite. I’d be surprised if you could even detect the change in illumination on the ground during the passage of the penumbra over your location.
No. In order to cast a dark shadow on you, the object’s apparent size (i.e. size as seen from you) must be larger than the sun, or at least comparable. From the ground, even a large satellite will look like a tiny dot crossing the sun, if it’s visible at all. Even the International Space Station appears as a tiny silhouette against the sun, as you can see here.
Of course, if the satellite is big enough – and the moon is a big enough satellite – then it will cast a visible shadow on the earth. There have been a number of pictures on the internet of this:
Dish Earth, currently dish network’s channel 287, will occasionally show the shadow of the moon. It is not very distinct. I don’t know if there is an optics explanation or if dish earth is just fuzzy.
I’d put my money on that. I live near an air corridor and have been swept with shadows like this numerous times. It’s kinda wierd because I would guess it happens at least once a month and I’ve never heard anyone mention it happening until this thread. Anyway, sometimes I can’t see the plane that did it.
To have the same angular diameter as the sun, a satellite orbiting at the same altitude as the ISS (350 km) would have to be about two miles across (3.2 km).
Thanks all for confirming my original suspicion - it couldn’t have been anything like a satellite.
Since I was near the corner of my yard I had a clear view of quite a bit of sky. Since I was so perplexed, I kept looking for the source for a good few minutes. It seems almost impossible to me that I would have missed an airplane.
A large bird is possible I suppose. If it was low and fast moving it might have been behind the neighbors’ house before I looked in the right direction. My immediate assumption was that it was a plane, so I looked high first.
It definitely felt funny though, to have what seemed like such a large shadow seemingly come out of nowhere.
When I was flying it was common to watch our shadow track across the ground. As described above, the penumbra was a bunch larger & less distinct than the umbra, which wasn’t all that distinct to begin with. But both were pretty obvious as darker patches on the ground. So they’d be perceived as a distinct shadow passage by any ground observer they passed over.
If you look up at a contrail, it’s often hard to pick out the generating aircraft as more than a speck. Absent the contrail, you’d have a very hard time spotting the aircraft. Consider that on a day with weather good for contrail formation you’ll see several, if not a dozen, being drug across the sky at any time, at least here in the central US. On another day you’ll see zero contrails. But the planes are still there; the airline’s schedules are pretty consistent day to day. And you’re very hard-pressed to spot any such planes unless the sun happens to glint off one.
Finally, looking near the sun really fries your vision. Something pretty big & obvious could be within a couple solar diameters and you’d not be able to spot it until your eyes recover in a couple minutes. By which time the aircraft is far enough from the sun that you don’t know where to look to spot it. And it isn’t something big and obvious.
Though the Sun is a large light source in this context, distant stars are not. Objects cast multiple shadows including one for each star. You’d never sense these shadows by looking for them on the ground, but if you are inside such a shadow and it moves away, you would be able to see the star suddenly appear in the sky where it had been dark before.
Astronomers call temporary star shadows that you detect from inside them this way “occultations”, and study asteroid movements and dimensions by carefully timing them. Amateur astronomers get into the act, too, and magazines like Sky & Telescope usually carry notices about interesting occultations that are soon to happen, including time and location for observing them.
