Astronomy Picture of the Day |
APOD: 2004 September 12 - Mercury: A Cratered Inferno
Explanation:
Mercury's surface looks similar to our Moon's.
Each is heavily
cratered and made of rock.
Mercury's diameter is about 4800 km, while the
Moon's is slightly less at about 3500 km
(compared with about 12,700 km for the
Earth).
But
Mercury is unique in many ways.
Mercury is the closest planet to the
Sun,
orbiting at about 1/3 the radius of the
Earth's orbit.
As Mercury slowly rotates, its surface temperature
varies from an unbearably cold -180 degrees
Celsius to an unbearably hot 400 degrees
Celsius.
The place nearest the
Sun in
Mercury's
orbit changes slightly each orbit - a fact used by
Albert Einstein
to help verify the correctness of his then
newly discovered theory of gravity:
General Relativity.
The above picture was taken by the only spacecraft ever to pass
Mercury:
Mariner 10 in 1974.
A new mission, Messenger,
launched for Mercury last month
and is scheduled to enter orbit around the Solar System's
innermost planet in in 2011.
APOD: 2003 April 12 - Mercury on the Horizon
Explanation:
Have you ever seen the planet Mercury?
Because
Mercury orbits so close to the Sun,
it never wanders far from the Sun in
Earth's sky.
If trailing the Sun,
Mercury will be visible
low on the horizon for only a short while
after sunset.
If leading the Sun, Mercury
will be visible only shortly before
sunrise.
So at certain times of the year an
informed skygazer with a little determination
can usually pick Mercury
out from a site with an unobscured horizon.
Above, a lot of determination has been combined
with a little
digital trickery to
show Mercury's successive positions during March of 2000.
Each picture was taken from the same location in Spain
when the Sun itself was 10 degrees below the
horizon and superposed
on the single most
photogenic sunset.
By the middle of this month, Mercury will again be well
placed for viewing above the western horizon at sunset,
but by the end of April it will have faded and dropped into the
twilight.
On May 7th,
Mercury
will cross the Sun's disk.
APOD: 2003 February 16 - Southwest Mercury
Explanation:
The planet Mercury resembles a moon. Mercury's old surface is heavily cratered like many moons.
Mercury is larger than most moons but smaller than
Jupiter's moon
Ganymede and
Saturn's moon
Titan.
Mercury is much denser and more massive than any moon,
though, because it is made mostly of iron. In fact, the
Earth is the only planet more dense.
A visitor to Mercury's surface
would see some strange sights.
Because
Mercury rotates exactly three times every two orbits around the
Sun, and because
Mercury's orbit is so elliptical, a visitor to
Mercury might see the
Sun rise, stop in the sky, go back toward the rising
horizon,
stop again, and then set quickly over the
other horizon.
>From Earth, Mercury's proximity to the
Sun causes it to be
visible only for a short time just after
sunset or just before sunrise.
Authors & editors:
Robert Nemiroff
(MTU) &
Jerry Bonnell
(USRA)
NASA Web Site Statements, Warnings, and
Disclaimers
NASA Official: Jay Norris.
Specific rights apply.
A service of:
EUD at
NASA /
GSFC
& Michigan Tech. U.