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Understanding Visible Phases of Lunar Eclipses

This is an overview of lunar eclipse phases sufficient to understand what is visible from Earth, and thus understand to what extent any particular eclipse (for example that of March 23, 5 B.C. which is often used to date the death of Herod the Great) is visible from some point on Earth (for example, Jerusalem, where Herod ruled).

Note the four example positions in diagram #1 below. Lunar eclipses (as well as solar eclipses) are caused by three fundamental phenomena, the:

  1. Earth orbits the Sun counterclockwise.
  2. Earth rotates counterclockwise about its polar axis which is orthogonal to the equatorial plane (plane through the Earth's equator)
  3. Moon orbits the Earth counterclockwise, but at a slant.

The notion of "counterclockwise" movement is from the perspective of looking "down" on the solar system from"above", i.e. looking down on the north pole of the Earth, Moon and Sun.

Note further that the orbit of the Moon is inclined (is slanted) approximately 5 degrees relative to the plane of the "ecliptic" which is the Earth's "flat" orbit around the Sun (it is called the "ecliptic" because that is the plane in which eclipses can happen). Consequently, most of every month the Moon is either above or below the ecliptic and can't be in the Earth's shadow at all (for example as shown at the left and right positions of diagram #1), and approximately twice a year the Moon both crosses the ecliptic (either from above or from below as in the bottom and top positions of diagram #1 respectively) and also passes through the Earth's shadow when a total lunar eclipse occurs. At other times of the year, the Moon passes partially through the Earth's shadow and partial lunar eclipses of varying degrees occur. Consequently, lunar eclipses can last from several minutes (when only the penumbral shadow is partially grazed) to about five and a half hours (a maximum total eclipse from penumbral eclipse begins, through totality, to penumbral eclipse ends). Lunar eclipses don't happen exactly every six months because the Moon's orbit around the Earth isn't exactly the same every month (or every six, or twelve, etc.) - some months are longer and some are shorter and so the Moon doesn't return to the same position after the same number of months.

Two different continuous movements affect visibility of a lunar eclipse:

  1. The Moon moves counterclockwise through the Earth's shadow.
  2. The Earth rotates ground-based observers counterclockwise in to, or out of, regions of eclipse visibility.

The net effect is movement 1 causes the lunar eclipse while movement 2 causes varying ground-based visibility of the eclipse at "terminators" (imaginary lines circling the Earth vertically, perpendicular to the ecliptic) at the moment of a change of phase in the lunar eclipse.

Diagram #1

[attribution for above diagram is unknown - please email below if owner can be identified]

Note four things in diagram #1:

  1. A lunar eclipse happens when the Moon, Earth and Sun are all three lined up and the Moon is in the earths shadow (a full moon).
  2. In actuality, the Earth is much farther from the Sun and the Moon farther from the Earth so that the Earth's shadow is actually a very long and narrow cone shape, and the Moon will pass though it (when they're all lined up) in only a few hours.
  3. The Earth is revolving counterclockwise around the Sun, and rotating on its axis counterclockwise, and the Moon is revolving around the Earth counterclockwise: all three motions are happening together all the time.
  4. A solar eclipse happens when the Moon, Earth and Sun are also all three lined up but the Earth is in the Moon's shadow (a new moon).

Diagram #2

"Lunar Eclipse Geometry" (from Mr. Eclipse.com)

Diagram #3

How on Earth Was This Image Made? (from NASA)


Diagram 2 shows what would be seen looking down on a lunar eclipse when the Sun, Earth and Moon are lined up in the top position of diagram #1. Note very carefully in diagram #2 "Lunar Eclipse Geometry" (from Mr. Eclipse.com) that you are looking down on the Earth's north polar region (the exact pole position is offset from center because the Earth's axis is tilted). There are additional illustrations at Science: Mechanics of Lunar Eclipses. The essential concept to grasp is the reason for three different kinds of lunar eclipse:

  1. Penumbral lunar eclipse when the Moon is in the Earth's penumbra where some of the Sun's light can still reach. Penumbral eclipses are not distinguishable by the unaided human eye; a light sensitive telescope as used by astronomers is needed.
  2. Umbral (or total) lunar eclipse when the Moon is in the Earth's umbra where none of the Sun's light can reach.
  3. Partial lunar eclipse (not shown in the diagram) when the Moon straddles the line between penumbra and umbra, partially in both areas.

Note also that as the Moon revolves counterclockwise around the Earth, the Moon moves through the eclipse in phases:

  1. starting the first entry into the Penumbra (contact position "P1" in diagram #4),
  2. then first entry in the umbra (contact position "U1" in diagram #4),
  3. then fully in the umbra (contact position "U2" in diagram #4),
  4. then emerging from the umbra into the opposite penumbra (contact position "U3" in diagram #4),
  5. then emerging from the opposite penumbra (contact position "U4" in diagram #4),
  6. lastly exiting fully from the opposite penumbra (contact position "P4" in diagram #4)

Now, imagine you are in a spaceship traveling parallel to the Earth's orbit, moving counterclockwise around the Sun, hovering on the night side of Earth and always centered in the Earth's shadow but inside the Moon's orbit, i.e. you're traveling around the Sun staying on the night side of the Earth, and when the Moon moves into a full Moon position, your spaceship is already in the Earth's shadow between the Earth and the Moon; if you look out your left window you see the night side of the Earth and the Sun is hidden behind the Earth, and if you look out your right window you see the full Moon. Keep this vantage point in mind as you read further.

From your spaceship, as you look down at the Earth you see (in diagram #3) the line between where the sunlight ends and shadow begins. This line is called the "terminator" because it is where the sunlight and darkness each terminate. Because the Earth is rotating counterclockwise, different areas of the Earth's surface rotate under the "terminator" - the terminator doesn't move, rather the Earth is rotating underneath the "fixed" terminator. What is seen then is a steady movement from left to right of the Earth's surface pass under the terminator from daytime into nighttime. We call that change "sunset" because people to the left (the west) of the terminator in diagram #3 see the Sun "set" as they rotate under the terminator from daylight into darkness.

Keep in mind this concept that the Earth's rotation causes "observers" at different locations to pass the terminator at different times. For example, ground-based observers west (or left) of the terminator are still in sunlight and can't see stars, whereas ground-based observers east (or right) of the terminator are in darkness and can see stars. Sunset and moonrise are observed at this terminator. Terminators completely circle the Earth vertically, perpendicular to the ecliptic (similar to lines of longitude on a globe), intersecting at the north and south polar regions. So, on the opposite side of the Earth (imagine the other side of diagram #3 showing the Pacific ocean) is the same terminator except that moonset and sunrise are observed.

Meanwhile, back in your spaceship, consider that as the Moon moves into the Earth's shadow (looking out the right side of your spaceship) a lunar eclipse phase terminator occurs on Earth (looking out the left side of your spaceship). Beneath you and your spaceship, the Earth is rotating ground-based observers counterclockwise towards or into the lunar eclipse viewing area. At the moment the Moon begins its very first contact with the Earth's penumbra (recall diagram #2, discussion point 3.a) at the western edge of the Earth is terminator "P1" (Moon begins to enter the Earth's penumbral shadow) where people further west (or left) in the daylight side can't see the beginning of the eclipse yet because the Earth hasn't rotated enough, but people further east (or right) in the nighttime side can see the lunar eclipse beginning. In fact, these people east of the western P1 terminator will see the entire eclipse. Similarly, after the Earth rotates a little more and when the Moon begins to enter the umbra (recall diagram #2, discussion point 3.b) again at the western edge of the Earth is terminator "U1" where observers further west can't yet see the Moon entering the Earth's umbral shadow but observers further east can.

Consider also what happens and is seen from the opposite (eastern) side of the Earth. There, the Earth is rotating ground-based observers counterclockwise away from or out of the lunar eclipse viewing area. When the western ground-based observers at western terminator "P1" were being rotated into the P1 portion of the lunar eclipse, the eastern ground-based observers at the eastern terminator P1 on the opposite side of the planet were being rotated away from the eclipse, never saw it start, and will never see any part of it because they are rotating counterclockwise further into the daytime side of the planet. But some eastern side-of-the-planet ground-based observers who stood further to the west of the eastern terminator P1, saw P1 begin. But as the planet rotates them also away from the eclipse, by the time the Moon begins to enter the umbra ("U1" starts) they'll miss that phase, i.e., they saw P1 but they'll not see any of U1, nor any more phases of the eclipse.

So, for every terminator P1, U1, U2, etc. on the western side of the planet (below which terminator ground-based observers are rotated further into seeing the remaining lunar eclipse phases) on the opposite eastern side of the planet there corresponds a continuation of the same terminator P1, U1, U2, etc., in the exact same sequence and timing (below which terminator ground-based observers are rotated further away from seeing any more of the remaining lunar eclipse phases) - the western and eastern terminator segments being actually one continuous terminator encircling the planet.

As the Earth rotates and as the Moon enters (and exits) its various eclipse phases, terminators on the western and eastern side of the planet 'mark' where the current phase of the lunar eclipse is visible or not. Ground based observers at the western terminators are rotated into seeing more of the eclipse phases, while ground based observers at the eastern terminators are rotated away from seeing more of the eclipse phases. Between the western and eastern terminators on the night time side of the planet all phases of the eclipse are visible, whereas on the opposite side of the planet between the eastern and western terminators no phases of the eclipse are visible.

So ground based observation areas may be classified in four groups, those on the:

Diagram #4, "Total Lunar Eclipse of -0004 March 23" (.pdf download) sometimes known as "Herod's Eclipse" is one of several Lunar Eclipses of Historical Interest, charted by Fred Espenak of NASA. Below is the same image previously rendered by Espenak as a .gif:

Diagram #4

(click to enlarge)

In the diagram "Total Lunar Eclipse of -0004 March 23" (where year -0004 equates to 5 BC), the:

An important point needs to be reiterated. Penumbral eclipses (type "N" in the table below, where the Moon is in the Earth's penumbra to which some sunlight still reaches) though "visible" to astronomers with sensitive telescopes are not discernible to the unaided human eye. The human eye does not readily distinguish the lesser amount of sunlight reflecting off a penumbral shadow. Only partial and total lunar eclipses were generally documented by ancient human observers because they did not have telescopes to distinguish the penumbral phases. The penumbral phases still happened, but human eyes couldn't distinguish the somewhat reduced reflected sunlight levels to recognize when those phases began and ended.

This means even though penumbral eclipse phases can now be calculated, ancient observers will not have recorded their times accurately, if at all, but they could note accurately the umbral and partial-umbral eclipse phases (assuming the Moon wasn't obscured by clouds on those nights).

Here are Fred Espenak's definitions of the lunar eclipse phase contact points (and their corresponding Earth terminators):

P1 - Instant of first exterior tangency of Moon with Penumbra (Penumbral Eclipse Begins)
U1 - Instant of first exterior tangency of Moon with Umbra (Partial Umbral Eclipse Begins)
U2 - Instant of first interior tangency of Moon with Umbra (Total Umbral Eclipse Begins)
U3 - Instant of last interior tangency of Moon with Umbra (Total Umbral Eclipse Ends)
U4 - Instant of last exterior tangency of Moon with Umbra (Partial Umbral Eclipse Ends)
P4 - Instant of last exterior tangency of Moon with Penumbra (Penumbral Eclipse Ends)

Fred Espenak, Key to Lunar Eclipse Global Maps
NASA/Goddard Space Flight Center

Below is a table of data for all lunar eclipses in the years 13 to 1 BC, excerpted from -0099 to 0000 at NASA's Five Millennium Catalog of Lunar Eclipses produced by Fred Espenak:

where the type of lunar eclipse can be (see Espenak's Key to Catalog of Lunar Eclipses for meanings of each column):

N = Penumbral Eclipse.
P = Partial (Umbral) Eclipse.
T = Total (Umbral) Eclipse.

         Local Circumstances at Greatest Eclipse:  -0012 to   0000

             Greatest    Saros          Pen.   Umb. S.D. S.D.  GST    Moon  Moon
      Date    Eclipse Type #    Gamma   Mag.   Mag. Par  Tot  (0 UT)   RA    Dec
                                                                 h     h      °
 -0012 Feb 21  13:01   P   52  -0.646  1.739  0.637  89m   -    9.9  10.15  10.9
 -0012 Aug 16  02:54   P   57   0.652  1.662  0.692  82m   -   21.5  21.52 -14.2
 -0011 Feb 09  12:42   T-  62   0.062  2.817  1.704 118m  52m   9.1   9.44  15.3
 -0011 Aug 05  19:16   T+  67  -0.063  2.751  1.764 108m  50m  20.8  20.85 -17.9
 -0010 Jan 29  15:04   P   72   0.761  1.514  0.440  74m   -    8.4   8.71  19.2
 -0010 Jul 26  07:30   P   77  -0.844  1.343  0.307  62m   -   20.1  20.14 -21.2
 -0010 Dec 20  13:10   N   44  -1.304  0.482 -0.519   -    -    5.8   5.80  22.4
 -0009 Jan 19  00:32   N   82   1.406  0.303 -0.716   -    -    7.7   7.95  22.4
 -0009 Jun 15  21:57   N   49   1.185  0.744 -0.346   -    -   17.4  17.35 -22.3
 -0009 Dec 10  04:46   P   54  -0.641  1.693  0.700  82m   -    5.1   5.02  22.3
 -0008 Jun 03  23:00   T   59   0.414  2.150  1.078 106m  22m  16.7  16.58 -21.9
 -0008 Nov 28  18:50   T-  64   0.026  2.842  1.811 109m  50m   4.4   4.23  21.5
 -0007 May 24  05:39   T   69  -0.373  2.199  1.179 104m  32m  16.0  15.83 -20.7
 -0007 Nov 18  03:04   P   74   0.750  1.541  0.452  75m   -    3.6   3.44  19.7
 -0006 Apr 14  12:05   N   41   1.409  0.270 -0.696   -    -   13.3  13.37  -7.2
 -0006 May 13  19:14   N   79  -1.100  0.841 -0.132   -    -   15.3  15.11 -18.9
 -0006 Oct 08  09:19   N   46  -1.571  0.051 -1.068   -    -    1.0   0.83   3.9
 -0006 Nov 07  04:15   N   84   1.462  0.254 -0.872   -    -    2.9   2.66  17.1
 -0005 Apr 04  04:57   P   51   0.710  1.559  0.581  77m   -   12.6  12.72  -3.9
 -0005 Sep 27  11:07   P   56  -0.836  1.377  0.301  63m   -    0.2   0.12  -0.0
 -0004 Mar 23  18:20   T+  61  -0.022  2.847  1.818 111m  51m  11.9  12.06  -0.4
 -0004 Sep 15  20:10   T+  66  -0.081  2.732  1.717 109m  50m  23.5  23.44  -3.8
 -0003 Mar 13  00:40   P   71  -0.797  1.454  0.367  70m   -   11.2  11.38   3.2
 -0003 Sep 05  11:06   P   76   0.622  1.720  0.743  84m   -   22.8  22.77  -7.2
 -0002 Jan 31  07:43   N   43   1.434  0.296 -0.812   -    -    8.5   8.85  19.2
 -0002 Mar 02  01:06   N   81  -1.529  0.123 -0.986   -    -   10.5  10.68   7.0
 -0002 Jul 27  18:44   N   48  -1.264  0.552 -0.445   -    -   20.2  20.27 -21.3
 -0002 Aug 26  03:32   N   86   1.326  0.432 -0.551   -    -   22.1  22.09 -10.4
 -0001 Jan 20  11:52   P   53   0.683  1.649  0.590  82m   -    7.8   8.07  21.3
 -0001 Jul 17  05:12   P   58  -0.554  1.882  0.832  95m   -   19.5  19.52 -22.6

  0000 Jan 09  23:08   Tm  63  -0.042  2.799  1.792 107m  50m   7.1   7.28  22.5
  0000 Jul 05  08:37   T-  68   0.209  2.535  1.445 115m  47m  18.7  18.74 -23.1
  0000 Dec 29  14:31   P   73  -0.708  1.568  0.579  76m   -    6.4   6.50  22.8

Fred Espenak, "Five Millennium Catalog of Lunar Eclipses -0099 to 0000 ( 100 BCE to 1 BCE )",
NASA's Goddard Space Flight Center

Diagram #4 was computer generated by Fred Espenak from the data for the entry above dated -0004 Mar 23, a total lunar eclipse.

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(last updated July 6, 2020)