What is your frame of reference for what to
measure for elevation from on the moon?
That’s an interesting question– one that
required a bit of research. On Earth, elevations
are reported relative to sea level, but there
are no seas on the moon. I found out that most
lunar elevations are reported relative to the
surface of sphere with a radius of 1738 km
(e.g., Margot et al., 1999). This means that if
you took an imaginary ball and lined-up the
center of the ball with the center of the moon,
the surface of the ball would be the reference.
Things like lunar volcanoes would stick up above
the surface of the ball and have positive
elevation. Things like lunar craters would be
below the surface and have negative elevation
(like “below sea-level” on Earth). The moon is
not actually a perfect sphere; it is a spheroid
or ellipsoid. This means that it is sort of egg
shaped and not perfectly round. Because it is
egg-shaped, a line from its center to its
surface (the radius) is going to be different
lengths in different directions (it will be
shorter from the center to the surface of the
narrow part of the egg and longer from the
center to the surface of the pointy part of the
egg). If you took the egg shaped moon and
squashed it back into a sphere, the sphere would
have a radius of 1738 km.
How does a compass perform in deep space?
What does it point to?
Best Regards. Also, you guys should watch
adventure time. It is majestic.
Well, I have never really thought of that
before. As a field-geologist I use a compass all
of the time (on Earth). A compass is made up of
magnetized material; many types of metal alloys
that contain iron can be magnetized. Because the
needle is magnetized, it responds to Earth’s
magnetic field. The magnetic field is potential
field. When the magnetized compass “sees” the
magnetic potential field, it will experience a
force and deflect. This will make the needle
point toward the north magnetic pole and away
from the south magnetic pole. Earth has a solid-
metal inner core that rotates in a molten
(liquid) outer core. This system, called the
geodynamo, is what created Earth’s magnetic
field. As you get farther and farther from Earth
(out into space), its magnetic field get weaker
So, how will a compass work in deep space?
A compass needle will always respond to a
magnetic field, but are there magnetic fields in
space? I think that there must be magnetic
fields everywhere in space, though they might be
caused by different things. If you are very far
away from a body that has a geodynamo (like a
planet with a molten core), the magnetic field
is not going strongly affected by it. Magnetic
fields are also generated by electrical
currents, which is basically the movement of
charged particles like electrons, protons, and
ions (atoms that have lost or gained some
electrons). This is basically how the geodynamo
is generated, but a different version of this
process might be important in space. Currents of
ions known as solar winds are very common in
space. The movement of these ions would cause a
magnetic field that a compass would respond to.
The magnetic field that is generated by a
current is always perpendicular to the current,
so the compass needle would point at a right
angle to the direction of the current.
There might be other important sources of
magnetic fields in space. You should check out
the NASA website (nasa.gov) for more information!
Margot, J.L., Campbell, D.B., Jurgens, R.F.,
Slade, M.A. (1999). Topography of the lunar
poles from radar interferometry: A survey of
cold trap locations. Science, 284, 1658–1660.
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