# In the shadow of the Sun

Gold Member

## Main Question or Discussion Point

Would a nearby Gamma Ray Burst (GRB) cause the Sun to cast a shadow on Earth if it were to shine from behind it?

Related Astronomy and Astrophysics News on Phys.org
UltrafastPED
Gold Member
What would you expect for the size of this shadow?

Gold Member
Hi, Ultra. I guess the shadow would be the size of the Sun, right?

mfb
Mentor
To a very good approximation, indeed.
What do you know about the relative size of sun and earth?

UltrafastPED
Gold Member
Yes, but what size is the shadow of the sun on the surface of the earth?

Chronos
Gold Member
As a plasma ball, the sun is a very efficient gamma ray shield. The sun moves across the sky an apparent 60 arc minutes per day. It's apparent diameter is about 30 arc minutes. You should be able to figure out from this about how long the sun could shield us from a GRB [a point source] occurring on a line of sight behind the sun.

Gold Member
To a very good approximation, indeed.
What do you know about the relative size of sun and earth?
The Sun is bigger but farther, and Chronos said it is 30 arc minutes in diameter.

Yes, but what size is the shadow of the sun on the surface of the earth?
30 arc minutes?

As a plasma ball, the sun is a very efficient gamma ray shield. The sun moves across the sky an apparent 60 arc minutes per day. It's apparent diameter is about 30 arc minutes. You should be able to figure out from this about how long the sun could shield us from a GRB [a point source] occurring on a line of sight behind the sun.
Ok. So what I basically want to know is if the visible luminosity of a GRB is powerful enough to cause the Sun to cast a shadow on Earth. I use GRBs as an example, but I guess a supernova could do it, too, right?

russ_watters
Mentor
The Sun is bigger but farther, and Chronos said it is 30 arc minutes in diameter.

30 arc minutes?
How big is the earth in arc minutes? How much of the earth does that cover...?
Ok. So what I basically want to know is if the visible luminosity of a GRB is powerful enough to cause the Sun to cast a shadow on Earth.
Gamma ray bursts aren't very bright visibly. So it wouldn't be much of a "shadow"
I guess a supernova could do it, too, right
Yes, and much brighter than a GRB. But still not much of a "shadow".

Last edited:
Gold Member
How big is the earth in arc minutes? How much of the earth does that cover...?
I don't know..

I'm trying to picture what the sky would look like if the Sun were to 'eclipse' a GRB or a supernova. It seems that the glare from either of those two events could overwhelm the visible brightness of the Sun by several orders of magnitude, no?

Are you saying the Sun wouldn't cast any shadow on the Earth with that much light in the background?

Last edited by a moderator:
Chronos
Gold Member
Radiation from a GRB behind the sun would be enormously diminished.

russ_watters
Mentor
I don't know..
It's a trick question; That doesn't translate into a size on earth. For this situation, the angular diameter of the sun doesn't matter, only the fact that its real diameter is much larger than earth. All of earth will be in "shadow"
I'm trying to picture what the sky would look like if the Sun were to 'eclipse' a GRB or a supernova. It seems that the glare from either of those two events could overwhelm the visible brightness of the Sun by several orders of magnitude, no?
No. Those events are nowhere close to as bright as the sun (er - perhaps a GRB from within our galaxy would be, but they are rare events and so far are only seen from far away). It depends on distance of course, but a supernova in our galaxy might shine as brightly as Venus.
Are you saying the Sun wouldn't cast any shadow on the Earth with that much light in the background?
Not in the normal sense of the word, no.
 Tionis, I apologize; I accidentally edited your post instead of replying (for moderators, the buttons are right next to each other). I've tried to restore it to its original form.

mfb
Mentor
Supernovae are bright, but they won't outshine the sun in visible light. The supernova of 1604 was at a distance of 20 000 light years and reached an apparent magnitude of -2.5. Our sun is 10^10 times brighter than that, in order to get the same magnitude a similar supernova would have to be closer by a factor of 100 000, or .2 light years away. There is no star in this distance, and I think all the stars within 10 light years won't explode as supernova.

Even if there would be some very bright source, the sun has no solid surface. There would be no shadow similar to a cloud, there would be some large (relative to clouds) transition region.

Gold Member
OK. I'm convinced. Thanks :-)