Listed below are questions submitted by users of "From Stargazers to Starships" and the answers given to them. This is just a selection--of the many questions that arrive, only a few are listed. The ones included below are either of the sort that keeps coming up again and again, or else the answers make a special point, often going into details which might interest many users.
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However.... you know that the Moon always presents Earth with the same face. If you read my section "The Moon--the distant view" you know that the reason is a slight elongation along the Earth-Moon line, and that the Moon's long axis slowly swings back-and-forth around the direction of Earth, like a pendulum ("libration"). I do not know the theory of those swings--they may be linked to the equatorial bulge of the Earth--but I vaguely recall an article in "Science," maybe 20-30 years ago, claiming that an asteroid
impact started them, even identifying the crater which that impact produced. The rotation of the Moon or the Earth contains much less energy than the orbital motion, it can be affected by a slanting blow, and the Moon is so much smaller than Earth, so THAT is possible.
Enjoy your physics, as well as other things that interest 14 year olds, and don't let your grades in other subjects slip!
A launch vehicle crawling upwards at 1 mph would be wasting an enormous
amount of thrust just to keep itself from falling! And even if you raise
the space vehicle slowly to (say) 1000 miles, to keep it there from
falling you still need give it an orbital velocity--less than 8000 m/s
because of the greater distance, but not that much less.
You might think that wings would be a more efficient way of keeping
the vehicle in the air--after all, the thrust of an airplane engine (in
cruising flight) may be only 5-10% of its weight. Unfortunately, this
efficiency drops very quickly above the speed of sound, and 8000 m/s is
about 24 times that velocity. Above a speed of several times the speed
of sound, the extra air resistance of the wings outweighs any advantage
they provide; it is better for the vehicle to quickly rise above the
dense atmosphere and avoid air resistance altogether.
In a stable orbit, with orbital velocity, gravity no longer threatens to bring
down the vehicle, it just determines its orbit. From that point
on, one can apply thrust at any rate. There is a story of a
communication satellite (I think of NASA's TDRSS system) which made
Earth orbit safely, but the engine which was to take it to its final
orbit at 42000 kilometers (6.6 Earth radii) failed. However the
spacecraft had plenty of on-board fuel, and a small motor meant to
adjust its orientation, which was able to tap that fuel supply. So over
the months that followed, guided by NASA controllers (the motor had to
be switched on and off to prevent overheating), it slowly limped to its
final station, reaching it safely.
That is probably as close as we have come to your "one mph" motion.
"Deep Space 1" with its ion engine is another such slowly accelerating
I understand that a year is the time between two successive vernal
equinoxes. In a year the earth will have orbited around the sun and the
earth's axis will have precessed a very little bit so that both the orbit
around the sun and the precession of the earth's axis go together to make
up the length of time between two successive vernal equinoxes. Now suppose
the earth's axis were not precessing. How long would a year be? How much
does the precession of the earth's axis affect the length of a year?
Thank you for your attention and any information will be greatly appreciated.
Let's first try a simple minded approach. The phenomenon is called
PREcession, so the spring equinox moves to a point a little EARLIER in
the Sun's journey around the zodiac. The location of the spring equinox
makes one circuit of the zodiac in 26000 years. Therefor, if the spring
equinox did NOT move to intercept the Sun on its trip around the sky,
the year (equinox to equinox, say) would be about (365 x 86400)/26000
seconds longer, or about 20 minutes.
But it's more complicated. What year do you have in mind? A CALENDAR
year extends from equinox to equinox, or from solstice to solstice. Most
people want holidays to stay with the right seasons, not migrate between
summer and winter (as Moslem ones do). If the precession were to stop,
the year in which holidays kept a fixed position that would be 20
On the other hand, if your field is celestial mechanics or
astronautics, "a year" is presumably the EARTH'S ORBITAL PERIOD around
the Sun. The orbital period does not depend on which way the Earth's
axis points in the sky--it is always the same, precession or no
precession (and it hardly varies over millions of years). So it is
always the longer of the two preceding ones.
A similar analogy holds for the day. Is it NOON TO NOON (24 hours
average) or is it the ROTATION PERIOD of the Earth around its axis? The
latter is 4 minutes shorter, because "noon to noon" includes a small
contribution from the shift of the Sun's position in the sky, about one
degree per day.
Ask a simple question... sorry about the complicated answer!
It is related to "the equation of time," a correction to sundial time
which must be applied (in addition to others) because the Earth orbits
the Sun in an ellipse, not a circle, and its speed in that orbit varies
around the year. The equation of time is mentioned in my site on the sundial
The POSITIONS of the north-south poles on the surface of the Earth may of course change if the entire crust of the Earth somehow slides around the interior, staying intact in the process. The theory of such "polar wandering" was briefly fashionable 50 years ago, but it no longer is. It is hard to observe any such motion if it is slow enough, but the magnetic signatures of lavas suggests that if the effect exists, is negligibly small.
To give you the argument, suppose you have samples of volcanic lavas
(which record the direction of the magnetic force as they harden) at a
location at latitude 40 North, from different eras. Then ancient
magnetizations will cluster either around the directions observed today,
or around directions opposed by 180 degrees. There are always some
deviations, ascribed to variations of the global field, but they are the
exceptions, not the rule. So, to misquote Kipling, "North is north and
south is south" even though MAGNETIC north and south may reverse by 180 degrees.