Are there different types of lunar eclipses?
Yes. The Earth's shadow has an 'umbra' and a 'penumbra'. If you are in the umbra and
look towards the Sun, the Earth blocks the entire Sun. If you are in the penumbra you
would see the Earth blocking only part of the Sun. Hence, there are three types of
(1) Total eclipses where the entire Moon is in the umbra.
During these, the only sunlight that reaches the Moon comes from the
glow of the light refracted and scattered from the Earth's atmosphere, which would look
like a fiery red ring everywhaere on the Moon. For this reason the Moon looks reddish during
a total eclipse. Most total eclipses last about an hour or so. (2) Partial eclipses occur when
only part of the Moon is in the umbra. The Moon looks like it has a chunk taken out of it.
Partial phases also bracket every total eclipse, each lasting about an hour, typically.
(3) Penumbral eclipses occur when the Moon misses the umbra entirely. From the Earth these
are subtle to detect, with only a weak dimming on one side of the full Moon. If you were on
that part of the Moon, the Earth would only block part of the Sun. Total eclipses
are by far the most interesting to observe.
How frequent are all lunar eclipses and total eclipses?
We usually get two or three, including all types, each year. On average we get about one total lunar eclipse
every year, but sometimes we'll get two in a year and sometimes none. Since total eclipses are
only visible from the side of the Earth that happens to be facing the Moon at the time, any
given location on the Earth will be able to see a total lunar eclipse about once every 2-3 years.
Why doesn't this occur every month as the Moon orbits the Earth?
The Moon's orbit is tilted about 5 degrees relative to the Earth's orbit around
the Sun. So usually the Moon either misses the shadow above or below as it orbits
the Earth. To line up the Sun, Earth and Moon well enough to get the Moon into
the Earth's shadow you have to have the Moon lie nearly in the plane of the Earth's orbit
around the Sun when the Moon is full. The points where the Moon crosses the
plane of the Earth's orbit are called nodes. Lunar nodes are aligned with the Sun and
Earth every 6 months, and during that time we get eclipses. In 2015, the
eclipse seasons are April and September.
What is a tetrad?
This is when four successive lunar eclipses are all total ones, without any partials or
penumbrals interspersed. Typically the total eclipses will not all be visible from
the same location, as the Moon will be below the horizon for some of the four.
Do eclipse seasons change?
Yes. For example, in 2017 the lunar and solar eclipses occurred in February and August, and by
2019 they've shifted to January and July. Eclipses occur a little earlier each year,
and return to their original starting point after about 18.6 years. This
`regression of the nodes' is caused by the lunar orbit wobbling like a top
around the Earth in response to the torques acting on it by the bulge of the
Earth and the pull of the Sun, both of which are inclined to the lunar orbital plane.
Does the fact that perigee occurs during eclipse make an eclipse special?
It does makes it very unusual. It occurred during the September 2015 eclipse, and will again
on Oct 8, 2033 (one saros period in the future).
In terms of how the eclipse appears though, a supermoon eclipse is not going to look a whole
lot different from a typical eclipse because the amount of the increased apparent
size of the Moon at perigee is rather minimal.
Distances and Orbital Shapes:
Interestingly, even though perigee occurs during the 9/27-28/15 eclipse, and perigee occurs
14 hours after the eclipse on 1/20-21/19, the eclipse on 1/20-21/19
is the closer of the two. How can this be? The main factor here is that the 1/20-21/19 eclipse
occurs around midnight for US observers. At midnight the full Moon is highest in the sky,
and the observer is closest to the Moon than during other hours of the night.
The 9/27/15 eclipse occurred in the early evening for US observers
and so was at a slightly larger distance than if the eclipse were to occur at mignight
(the Earth's radius is about 6400 km, about 2% of the distance to the Moon, so the effect
of different locations on the lunar distance is typically about 1%).
However, there is another more interesting effect on the distances that has to do with the lunar orbit. The eccentricity
of the Moon's orbit has many periodic variations, and gets more eccentric when the major
axis of the Moon's elliptical orbit aligns with the Sun. Have a look at the instantaneous
eccentricity (part of what is known as the osculating elements of the orbit) of the
lunar orbit from Figure 4.6,
again from the excellent NASA site. The closest approaches of the Moon to the Earth
occur when the eccentricity is highest and the orbit is more elliptical. Even so, the
eccentricity of the lunar orbit is never higher than about 0.078, so the maximum variation in
the apparent lunar diameter is never more than about 15%, and can be as low as 5% at times
when the orbit is most circular. This difference in size is noticeable, but not dramatic.
According to this
lunar calculator, the 1/20-21/19 perigee is actually slightly farther away (357344 km)
than the 9/27-28/15 perigee (356876 km). There are slightly closer perigees, for example
on Nov 14 2016 (356511 km; angular size 34.1 arcminutes when viewed from the optimal
location on the Earth, in this case near Hawaii), but no eclipse occurs during that full Moon
because the lunar nodes do not align with the Sun during November of that year.
All this is discussed in my supermoon article.
In celestial mechanics one can describe the lunar motion in terms of a stable orbit that undergoes
a perturbation by a third body (either the Sun or the bulge of the Earth). The equations of motion are
then reformulated in terms of a perturbation function. The orbit is described
by 6 parameters, two for shape (a = semi-major axis, and e = eccentricity), three for the orientation
of the ellipse in space (i = inclination, node = location of node, and omega = location of perigee),
and one to place the object on the orbit (tau = time of closest approach). One can solve the
perturbation equations for each orbital parameter and determine secular, short-term-periodic,
and long-term-periodic components to the equations. The NASA graph shows that these perturbations generate periodic changes in
the eccentricity, with rather intricate variations with time. The Moon's orbit is quite tricky to get right!
Fun fact: The eclipse on Jan 1, 2048 is actually ongoing at midnight. So on that
date we can ring in the new year with a total eclipse of the Moon!