LUnar eclipse questions

After last night's spectacular eclipse, I have some questions for you really
smart guys about lunar and solar eclipses.

1. How do we predict eclipses? (I know, the computer does it.) Seriously,
how do we do it? If possible, don't use calculus in your explanation.

2. How far back in history can we go to find people who accurately and
regularly predicted eclipses? I hear all the time about how "the ancients"
predicted eclipses. So how did they do it?

3. Why don't we have an eclipse each month? After all, a lunar eclipse is
caused when the earth comes between the moon and the sun and a solar eclipse
happens when the moon gets between the observer and the sun. Because the
moon orbits around the earth, it seems as though once each month we should
have a lunar and a solar eclipse. I suspect the answer has something to do
with the relative motions of the earth and moon because the both are moving
relative to the sun and relative to each other. Or, is it that there is a
solar and lunar eclipse each month but they are visible from earth only now
and then and most of the time are not visible on earth because the shadow
falls out in space somewhere. (Which raises the question: If an eclipse
happens and the shadow falls where no one can see it, did it happen?)

4. If we were on Jupiter looking at earth through our 8-inch Dobs, I assume
that a lunar eclipse on earth would look to us the same as a transit of
Jupiter by its Galilean moons and their shadow(s) looks to us on earth when
we observe Jupiter?

Thanks.

--
----
JAS

"JAS" wrote in message
...
After last night's spectacular eclipse, I have some questions for you
really
smart guys about lunar and solar eclipses.

1. How do we predict eclipses? (I know, the computer does it.)
Seriously,
how do we do it? If possible, don't use calculus in your explanation.

No calculus, but lots of spherical trigonometry. Basically, if you know
where orbits are, you can figure out where they intersect. For details I
recommend the book "Practical Astronomy With Your Calculator" by
Duffett-Smith, published by Cambridge Univ. Press.

2. How far back in history can we go to find people who accurately and
regularly predicted eclipses? I hear all the time about how "the
ancients"
predicted eclipses. So how did they do it?

Apparently some eclipse prediction went on in prehistoric times, e.g, at
Stonehenge. Without trigonometry, you can still predict eclipses to some
extent if you simply note that they recur at specific intervals (e.g., the
18-year Saros cycle).

3. Why don't we have an eclipse each month? After all, a lunar eclipse is
caused when the earth comes between the moon and the sun and a solar
eclipse
happens when the moon gets between the observer and the sun.

The moon's orbit and the earth's orbit (around the sun, or sun's orbit
around the earth if you look at it that way) are not in the same plane. If
you draw lines on the sky, the sun's path (the ecliptic) is a great circle,
and the moon's path is another great circle inclined slightly to the
ecliptic.

If the orientation of the two orbits were fixed, eclipse prediction would be
simple: we would have an eclipse whenever a full moon or a new moon happened
to fall at the place where the orbits cross.

The truth is a little more complicated. The plane of the moon's orbit is
rotating (revolving) relative to that of the sun (or earth). (I'm adopting
an earth-centered point of view for ease of explanation here.) That is, the
"ascending node" and "descending node" (where the orbits cross) are
themselves "orbiting" around the earth. They're a moving target, which is
why we don't have eclipses at the same time of year every time we have them.

But even this is not too hard to keep track of. The basic mathematics was
well understood by Ptolemy in ancient Greek times. (I have seen a
reproduction of a very nice eclipse diagram drawn by William of Conches
around 1150 A.D., by which time it was very old news.) What they lacked was
precise knowledge of the positions and orbital speeds. Ptolemy did the best
he could, but precise prediction of solar eclipse tracks in a 19th-century
development.

4. If we were on Jupiter looking at earth through our 8-inch Dobs, I
assume
that a lunar eclipse on earth would look to us the same as a transit of
Jupiter by its Galilean moons and their shadow(s) looks to us on earth
when
we observe Jupiter?

A solar eclipse would, but you'd need to be on Venus, not Jupiter, in order
to be able to look at the sunlit side of the earth.

During a lunar eclipse, the moon is above the dark side of the earth, and if
you viewed it from Jupiter's orbit you wouldn't see much.

We do see Jupiter's lunar eclipses. That is, we see the satellites pass
into Jupiter's shadow. For this to be visible, we have to be viewing
Jupiter from an angle, so we can see behind it.


--
Clear skies,

Michael Covington -- www.covingtoninnovations.com
Author, Astrophotography for the Amateur
and (new) How to Use a Computerized Telescope

3. The Moon's orbit is tilted slightly with respect to the Earth's. Most
months it either passes above or below the Sun's disk or the Earth's shadow.

4. No, as seen from Jupiter a _solar eclipse_ would appear as a transit of
the Moon's shadow across the face of the Earth. In fact, you should be able
to find some satellite images that show this quite nicely. A lunar eclipse
here would appear as...well, an eclipse when viewed from Jupiter. Like when
one of the Galilean moons disappears in Jupiter's shadow.



"JAS" wrote in message
...
After last night's spectacular eclipse, I have some questions for you
really
smart guys about lunar and solar eclipses.

1. How do we predict eclipses? (I know, the computer does it.)
Seriously,
how do we do it? If possible, don't use calculus in your explanation.

2. How far back in history can we go to find people who accurately and
regularly predicted eclipses? I hear all the time about how "the
ancients"
predicted eclipses. So how did they do it?

3. Why don't we have an eclipse each month? After all, a lunar eclipse is
caused when the earth comes between the moon and the sun and a solar
eclipse
happens when the moon gets between the observer and the sun. Because the
moon orbits around the earth, it seems as though once each month we should
have a lunar and a solar eclipse. I suspect the answer has something to
do
with the relative motions of the earth and moon because the both are
moving
relative to the sun and relative to each other. Or, is it that there is a
solar and lunar eclipse each month but they are visible from earth only
now
and then and most of the time are not visible on earth because the shadow
falls out in space somewhere. (Which raises the question: If an eclipse
happens and the shadow falls where no one can see it, did it happen?)

4. If we were on Jupiter looking at earth through our 8-inch Dobs, I
assume
that a lunar eclipse on earth would look to us the same as a transit of
Jupiter by its Galilean moons and their shadow(s) looks to us on earth
when
we observe Jupiter?

Thanks.

--
----
JAS

"Michael A. Covington" wrote
in message ...
"JAS" wrote in message
...
After last night's spectacular eclipse, I have some questions for you
really
smart guys about lunar and solar eclipses.

1. How do we predict eclipses? (I know, the computer does it.)
Seriously,
how do we do it? If possible, don't use calculus in your explanation.

No calculus, but lots of spherical trigonometry. Basically, if you know
where orbits are, you can figure out where they intersect. For details I
recommend the book "Practical Astronomy With Your Calculator" by
Duffett-Smith, published by Cambridge Univ. Press.


Thank you, sir. I'll track down the book.

2. How far back in history can we go to find people who accurately and
regularly predicted eclipses? I hear all the time about how "the
ancients"
predicted eclipses. So how did they do it?

Apparently some eclipse prediction went on in prehistoric times, e.g, at
Stonehenge. Without trigonometry, you can still predict eclipses to some
extent if you simply note that they recur at specific intervals (e.g., the
18-year Saros cycle).

And where do I read about the "Saros cycle?" Do eclipses appear in a
cycle -- e.g., every 18 years there will be an eclipse at the same time in
the annual cycle?

Thanks.


--
----
JAS




3. Why don't we have an eclipse each month? After all, a lunar eclipse
is
caused when the earth comes between the moon and the sun and a solar
eclipse
happens when the moon gets between the observer and the sun.

The moon's orbit and the earth's orbit (around the sun, or sun's orbit
around the earth if you look at it that way) are not in the same plane.
If
you draw lines on the sky, the sun's path (the ecliptic) is a great
circle,
and the moon's path is another great circle inclined slightly to the
ecliptic.

If the orientation of the two orbits were fixed, eclipse prediction would
be
simple: we would have an eclipse whenever a full moon or a new moon
happened
to fall at the place where the orbits cross.

The truth is a little more complicated. The plane of the moon's orbit is
rotating (revolving) relative to that of the sun (or earth). (I'm
adopting
an earth-centered point of view for ease of explanation here.) That is,
the
"ascending node" and "descending node" (where the orbits cross) are
themselves "orbiting" around the earth. They're a moving target, which is
why we don't have eclipses at the same time of year every time we have
them.

But even this is not too hard to keep track of. The basic mathematics was
well understood by Ptolemy in ancient Greek times. (I have seen a
reproduction of a very nice eclipse diagram drawn by William of Conches
around 1150 A.D., by which time it was very old news.) What they lacked
was
precise knowledge of the positions and orbital speeds. Ptolemy did the
best
he could, but precise prediction of solar eclipse tracks in a 19th-century
development.

4. If we were on Jupiter looking at earth through our 8-inch Dobs, I
assume
that a lunar eclipse on earth would look to us the same as a transit of
Jupiter by its Galilean moons and their shadow(s) looks to us on earth
when
we observe Jupiter?

A solar eclipse would, but you'd need to be on Venus, not Jupiter, in
order
to be able to look at the sunlit side of the earth.

During a lunar eclipse, the moon is above the dark side of the earth, and
if
you viewed it from Jupiter's orbit you wouldn't see much.

We do see Jupiter's lunar eclipses. That is, we see the satellites pass
into Jupiter's shadow. For this to be visible, we have to be viewing
Jupiter from an angle, so we can see behind it.


--
Clear skies,

Michael Covington -- www.covingtoninnovations.com
Author, Astrophotography for the Amateur
and (new) How to Use a Computerized Telescope

"JAS" wrote in message
...
"Michael A. Covington"
wrote
in message ...

No calculus, but lots of spherical trigonometry. Basically, if you know
where orbits are, you can figure out where they intersect. For details
I
recommend the book "Practical Astronomy With Your Calculator" by
Duffett-Smith, published by Cambridge Univ. Press.

Thank you, sir. I'll track down the book.

Glad to be of service! If you don't actually need to do the calculations,
any college-level astronomy book may be more illuminating. Duffett-Smith's
book more or less assumes you know the astronomy and just need the
algorithms.

Apparently some eclipse prediction went on in prehistoric times, e.g, at
Stonehenge. Without trigonometry, you can still predict eclipses to
some
extent if you simply note that they recur at specific intervals (e.g.,
the
18-year Saros cycle).

And where do I read about the "Saros cycle?" Do eclipses appear in a
cycle -- e.g., every 18 years there will be an eclipse at the same time in
the annual cycle?

Approximately, yes. It's not a perfect recurrence (the period and position
are slightly off). Books on ancient astronomy will tell you more.

Clear skies,
Michael