[extropy-chat] Question for Amara

Hal Finney hal at finney.org
Mon Apr 11 21:00:21 UTC 2005

Henrique writes:
> Amara, since you're an astronomer, I think you would be the most
> appropriate person to ask.  A hypothetical question. If our planet had
> rings, how would (if...) they be viewed from the surface? Would they
> glow at night? Would they be dark bands in the sky? Would they shadow
> the sunlight and affect the climate?

I am not Amara but I will offer some speculations.  Suppose we had a
Saturn-like ring system around our planet.  What would it look like?

The rings would probably orbit in a plane near to but not exactly the
same as the equator.  This would cause their appearance to be different
from different spots on the Earth.  Directly beneath the ring plane,
they would not be visible because they are too thin from that angle.
>From the more distant latitudes, they would be viewed from an oblique
angle and should be relatively visible.  From the poles they might not
be visible though, being below the horizon.

Generally, the ring plane and the equatorial plane would both be fixed
relative to the distant stars.  However, if this hypothetical Earth,
like ours, had inclined poles, the appearance of the rings would change
during the year.

There would be two factors.  One is the angle of the ring plane with
respect to the ecliptic, the plane of the Earth's orbit around the sun.
This would cause the sun's angle with respect to the rings to change
during the year.  When the sun was aligned with the ring plane, they
would be invisible, which would happen twice a year.  3 months later the
sun would be at maximum angle from the ring plane and the rings would
be at their brightest.

The other factor arises if the ring plane is different from the equatorial
plane.  During the course of the year, the time of day at which a given
observer is at maximum angle from the rings would change.  So at some
times of year the rings would be more visible at sunrise and at other
times they would be more visible at sunset.  These two year-based effects
would interact in a complicated way causing the ring's visibility from
different points on the Earth to vary from season to season.

I'd imagine that the rings would not be visible during mid day, the sun
being too bright.  They would be most noticeable just before sunrise and
after sunset, when much of the rings would be illuminated overhead.
If the rings are as big as Saturn's and extend several planetary
diameters out, then portions of them should be visible all night long.
The Earth's shadow will block out the overhead portion at midnight,
but parts should still be visible rising in the east and the west,
at least in middle latitudes.

The rings would be whatever color the dust is they are made of, probably
grayish like the moon.  Against the dark night sky they would look bright
white and colorless, like a full moon overhead.  At the horizons they
would shade into an orangeish color.

I suspect that the shadowing effects of the rings would have some limited
localized affect on climate.  Certain latitudes might consistently
find that their noontime winter sun had to shine through the rings.
Even if the rings were low density and this effect were small, it could
still change the temperature profile and cause localized adaptations by
planet and animal life.

All in all the rings would be a remarkable sight, one which would probably
have made a big difference in the historical course of understanding
astronomy.  The Earth's shadow would be visible every night in a very
obvious way, more so than the occasional lunar eclipse.  The change in
visibility with different latitudes would also be more conspicuous than
the variation in sun angles we are stuck with, and would point to the
roundness of the Earth.


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