& Lunar Eclipses
Understanding the Seasons
Time & Its Measurement
- Some Basic Information About Eclipses
||221,000 mi (356,000 km)
||252,000 mi (406,000 km)
||91.5 million mi (147 million km)
||94.5 million mi (152 million km)
- Different Types of Eclipses
- Total: Sun and Moon
- Partial: Sun and Moon
- Annular: Sun only
- Penumbral: Moon only
- Necessary Parameters for the Occurance of Eclipses
- The moon must be either new or full
- The moon must be near or at one of its two crossing positions
with respect to the ecliptic. The word node is a general
term which is used to express the intersection of two planes
in space. In other words, the moon must be at or near one
of its nodical positions with the ecliptic
- Circumstances Necessary for the Repetition of Two Eclipses
- The moon must return to a full or new phase.
The interval of time it takes the moon to complete its series
of phases is called its synodic period, 29.5306 days.
This is the driving beat for the repetition of solar and lunar
- Sidereal period: The period of revolution of
the moon around the earth. This equals 27.321661 days.
- The synodic period of the moon is different than
the lunar sidereal period. After one revolution of the
moon around the earth, the earth has completed approximately
1/12th of its orbital circuit around the sun. The earth,
moon, and sun are no longer in the same phase alignment.
To repeat the same alignment, the moon must continue in
its orbit for another 2 1/6 days. When this interval is
added to the sidereal period of 27 1/3 days, the synodic
period of 29 1/2 days is realized.
- The moon must be at or near one of its nodes.
This is called the moon's nodical period and it is
the interval of time it takes the moon to make two successive
crossings of either the ascending or descending node. At the
ascending node the moon crosses the ecliptic moving in a northerly
direction. Just the opposite is true for the descending node
where the moon crosses the ecliptic moving southward.
- Nodical period = 27.2122 days. This period is
shorter than the lunar sidereal period.
- The nodes move westward. Each successive time
that the moon returns to the same node, it will cross
at a position slightly west of its original location.
This westward motion of the nodes along the ecliptic plane
is referred to as the regression of the moon's nodes,
and it is why the nodical period is shorter than the sidereal
period. Remember, the moon is moving eastward amongst
the stars because of its orbital motion. At the same time
the nodes are moving westward, in the opposite direction
of the moon's orbital motion. The moon must first return
to the node before completing one orbit around the earth.
A complete nodical regression cycle takes 18.61 years.
- Why do the nodes regress? The regression of the moon's
nodes results from the sun's gravity trying to pull the
orbital plane of the moon into the plane of the ecliptic.
The result of this force however, acts at a right angle
to it, thus causing the moon's orbit to wobble (regress
or precess) to the west. One complete wobble takes 18.61
- In order for the repetition of an eclipse to occur, the
same number of days must be contained within integral
numbers of synodic and nodical months. Integers
are whole or counting numbers such as -2, -1, 0, 1, 2, etc.
47 synodic months = 51 nodical months =
1387.822 days REPETITION
The numbers 47 and 51 are integers, while the number of whole
days within 47 synodic months equals the same number of days
in 51 nodical months. This period of time is equivalent to
3.8 years and represents an eclipse cycle which does not necessarily
result in the repetition of similar eclipses. A cycle
of similar eclipses would represent a series of eclipses of
the same type, i.e., total, annular, etc.
- Eclipses occur more frequently than once every 3.8 years
because there are many different cycles which are running
- There must be at least two solar eclipses and two lunar
eclipses (including penumbral) happening in any given
- There can be as many as seven lunar and solar eclipses
taking place within a one year period with the maximum
number of any one type as great as five. Therefore, if
seven eclipses occurred during a year's time, and five
of them were solar, than only two could be lunar.
- Circumstances for the repetition of similar eclipses. These
criteria are used mainly for solar eclipses but they are also
applicable for lunar eclipses.
- The moon must return to the same phase,
new or full.
- The moon must be at or near a node.
- The moon must be at a similar distance from the earth.
This creates the repetition of the same type of eclipse.
- Line of apsides (ap-see-dez): The major axis
of the moon's elliptical orbit. It is the longest
line segment that can be placed within the boundary of
an elliptical orbit. For the moon, the line of apsides
completes one revolution around the sky in a period of
8.85 years. It revolves eastward in the plane of
the moon's orbit.
- Anomalistic month: The time interval between
two successive perigee (closest) or apogee (farthest)
passages of the moon. The period of time is 27.555
- The Anomalistic month is longer than the sidereal
month: Since the line of apsides makes one complete revolution
in a period of 8.85 years, the perigee and apogee positions
are continuously changing their direction in the sky.
The motion is towards the east, in the same direction
that the moon is revolving around the earth. Therefore,
each successive perigee or apogee position is slightly
ahead of its last location, and the moon must complete
one full revolution (sidereal period) around the earth
plus travel the additional distance that the apsis has
moved during the sidereal period. It must take the moon
a longer interval of time to complete this cycle than
the sidereal month.
- The revolution of the line of apsides results from solar
tidal forces (differential gravitation) which act to slow
down and speed up the moon in various parts of its orbit.
- Saros: To meet the conditions of a similar (solar)
eclipse, integral numbers of synodic, nodical and anomalistic
months must contain the same number of whole days. The result
is the saros, a period of 18 years, 10 or 11 days
(depending upon the number of leap years during that period)
in which an eclipse with similar circumstances will repeat
- 223 synodic months = 6586.321
- 247 nodical months =
6585.357 days = ECLIPSE
- 239 anomalistic months = 6585.538 days REPETITION
- The major beat of the saros is represented by the
synodic period. Similar eclipses will repeat themselves
in intervals of 6585 days. However, the occurrence of the
eclipse will be 1 / 3rd day later (0.321 day) shifting its
location approximately 120 degrees to the west of its previous
- The difference between the synodic and nodical periods,
0.036 day, will cause the position of the node to shift with
respect to the location of the sun. This will cause an eclipse
path to change with respect to latitude. A saros cycle begins
with eclipses occurring at one of the poles. Gradually, over
the period of the saros, eclipse paths migrate towards the
opposite pole, ending a particular saros series in that location.
- The residual in the anomalistic period when compared to
the synodic period (0.217 day) will cause a gradual shift
in the distance of the moon from the earth during a saros
cycle. This change will effect the type of eclipse which will
transpire during the saros cycle. Partial eclipses may blend
into annular events, then annular-total eclipses, then total
eclipses. The cycle will then reverse itself as the saros
continues to completion.
- Future total solar eclipses visible from Allentown or within
100 miles of the city
- May 01, 2079 -- occurs at sunrise
- October 26, 2144
- April 14, 2200
- June 07, 2263 -- occurs at sunset
- September 12, 2444
- Major solar eclipses visible until 2025
- August 21, 2017 -- total, across entire U.S. (Georgia)
- April 08, 2024 -- total, across U.S. (New England)