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FACTS ABOUT SPACE
1. Moon
craters
2.
Cosmic Rays 3.
Messier's list
4.
Meteors
5.
Parsecs
6.
Stars from the moon
7.
Billions and billions 8.
Lunar eclipses 9.
How long is a day?
10.
Moon rotation 11.
Earth's rotation 12. Earth's revolution
1. Did
you ever wonder why
there are so many craters on the moon, but so few on earth?
In the early solar system the
earth was bombarded with just as many
asteroids as the moon was. The difference is that the earth has water
and an atmosphere. Over the years the wind and water gradually
weathered away any signs of those original craters. The moon has no
atmosphere and no water, so once a crater forms there, it lasts
forever.
2. Cosmic rays
from space are always hitting the earth! But what are cosmic rays?
Cosmic rays come mainly from
exploding stars, known as supernovas. When
these stars explode they eject pieces of atoms that travel through
space at the speed of light. Fortunately, our atmosphere shields us
from most of these deadly rays. But next time you go into space, watch
out!
3. Did you ever
hear of galaxies and other space objects described as M1 or M86 and
wonder what the "M" stands for?
The "M" stands for Messier,
who was a French astronomer in the 1700's.
He prepared a list of faint astronomical objects known as "Messier's
list". The objects on the list were galaxies, nebulas, and globular
clusters, but Messier didn't know that at the time. He didn't know what
they were. So he just numbered them (M1, M2, etc.). Now we know that M1
is the Crab nebula and M86 is a galaxy in Ursis major. However, many
objects on the list are still referred to by their "M" number.
4. Rocks from
space are sometimes called meteors and sometimes called meteorites.
Which is correct?
It depends on where the rock
is. If it's burning up in the atmosphere,
like during a "meteor" shower, it's called a meteor. But if it survives
its journey through the atmosphere and is found on the ground, it's a
meteorite. What if it's flying through space? That would be a
called a meteoroid.
5. On Star Trek
they're always talking about distance in "parsecs". What is a parsec?
A parsec is 3.26 light years
(the distance light would travel in 3.26
years). The word parsec stands for "parallax second". Parallax is a
method of estimating the distance of a star from earth by viewing it
from two different spots in the earth's orbit and measuring the
difference in the angle to the star. A star is one parsec away if the
difference in the angle is one "second" of arc, which is equal to less
than one-millionth of a circle.
6.
Would the
star constellations look different if you were on the moon?
The answer is both yes and no. All the stars that can be seen from
earth would be visible from the moon, but since the moon has no
atmosphere you would see twice as many stars. The major features of the
constellations could still be made out, but the finer details would be
completely obscured by all of the additional visible stars. So the
night sky would look very different.
7.
As Carl Sagan
liked to say, there are billions and billions of stars in a galaxy, and
billions and billions of galaxies in the universe. But exactly how many
is a billion?
The answer seems obvious. A billion is a thousand millions, or a 1
followed by 9 zeroes (1,000,000,000). But someone from England or
Germany would disagree. In those countries a billion is a million
millions, or a 1 followed by 12 zeroes (1,000,000,000,000). If you're
like most people and find this hard to believe, just look up the word
"billion" in the encyclopedia and see for yourself!
8.
A lunar eclipse occurs when the moon, in its orbit around the earth
comes between the sun and the earth, casting a shadow on the earth. If
this is true, then why isn't there a lunar eclipse every month?
There would be an eclipse every month if the moon orbited around the
earth in the same plane that the earth orbits the sun. But it doesn't.
The moon's orbit is tilted by a little over 5 degrees relative to the
earth's orbit. Therefore, an eclipse only occurs when the intersection
of the orbital planes (the "nodes") happen to be in the earth-sun line.
9.
Everyone knows that a day is 24 hours, right? But that depends on what
you mean by a day.
The "solar" day, the time from when the sun appears in the same spot
overhead, averages 24 hours. But since the earth's orbit is elliptical,
the day is slightly shorter when the earth is closer to the sun (and is
consequently moving faster) and is slightly more than 24 hours when the
earth is at its farthest point. However, the day can also be measured
by the position in the sky of a particular star. This is called the
"sidereal" day, and it's only 23 hours, 56 minutes long. Why is the
sidereal day shorter than the solar day? Because in the time it takes
for the earth to rotate, the earth has moved a little in its orbit
around the sun.
10.
Why do we only see one side of the moon?
A long time ago the moon did rotate, and although there were no humans
to see it, all sides of the moon were visible. However, over time the
pull of the earth's gravity gradually slowed down the moon's rotation
until finally only one side faced the earth. The reason this happens is
that the earth's gravitational pull on the near side of the moon is
greater than on the far side. This uneven pull slightly distorts the
moon's shape and causes friction as it rotates. These "tidal forces"
are what cause the rotation to slow down.
11.
Are days getting longer?
Surprisingly, the answer is yes! But it's not noticable. Scientists
estimate that the Earth's rotation is slowing down at a rate of only
2.2 seconds every 100,000 years. The
gravity of
the moon, the same force that causes the tides, is gradually slowing
the earth's rotation. Eventually it will stop, for the same reason that
the moon's rotation long ago ceased. Interestingly, at the same time,
due to something known as the "Law of Conservation of Angular
Momentum", the earth and moon are also getting farther apart.
12.
Why does the earth revolve around the sun instead of vice versa?
It has to do with the fact that the mass of the sun is greater than the
mass of the earth. To be technical, the earth does not actually revolve
around the sun. Both the earth and the sun revolve around the center
of gravity of the earth-sun system. It appears that the earth revolves
around the sun because the sun is so much more massive. Consequently,
the center of gravity is actually inside the sphere of the sun (but not
at its center). The sun revolves around this point, tracing out a small
circle. When this type of "wobble" is detected when observing other
stars, it is an indication that the star has a planet revolving around
it.
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