Table of Contents

Solar & Lunar Eclipses

Intro to Astronomy

Equitorial Coordinates
Understanding the Seasons

Time & Its Measurement


The Solar System

The Earth

The Moon

Mecury, Venus, Mars

The Outer Planets

Solar System Debris

The Sun

Evolution of Stars

Intersteller Matter

Sky Literacy

  1. Some Basic Information About Eclipses
    1. Object
      Angular Diameter
      Moon Perigee 221,000 mi (356,000 km)
      33' 30"
        Apogee 252,000 mi (406,000 km)
      29' 31"
      Sun Perigee 91.5 million mi (147 million km)
      32' 30"
        Apogee 94.5 million mi (152 million km)
      31' 28"
    2. Different Types of Eclipses
      1. Total: Sun and Moon
      2. Partial: Sun and Moon
      3. Annular: Sun only
      4. Penumbral: Moon only
    3. Necessary Parameters for the Occurance of Eclipses
      1. The moon must be either new or full
      2. 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
    4. Circumstances Necessary for the Repetition of Two Eclipses
      1. 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 eclipses.
        1. Sidereal period: The period of revolution of the moon around the earth. This equals 27.321661 days.
        2. 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.
      2. 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.
        1. Nodical period = 27.2122 days. This period is shorter than the lunar sidereal period.
        2. 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.
        3. 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 years.
      3. 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 =    ECLIPSE
        1387.938 days            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.
      4. Eclipses occur more frequently than once every 3.8 years because there are many different cycles which are running concurrently.
        1. There must be at least two solar eclipses and two lunar eclipses (including penumbral) happening in any given year.
        2. 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.
    5. Circumstances for the repetition of similar eclipses. These criteria are used mainly for solar eclipses but they are also applicable for lunar eclipses.
      1. The moon must return to the same phase, new or full.
      2. The moon must be at or near a node.
      3. The moon must be at a similar distance from the earth. This creates the repetition of the same type of eclipse.
        1. 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.
        2. Anomalistic month: The time interval between two successive perigee (closest) or apogee (farthest) passages of the moon. The period of time is 27.555 days.
        3. 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.
        4. 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.
      4. 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 itself.
        1. 223 synodic months      = 6586.321 days     SIMILAR
        2. 247 nodical months       = 6585.357 days =  ECLIPSE
        3. 239 anomalistic months = 6585.538 days     REPETITION
      5. 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 path.
      6. 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.
      7. 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.
    6. 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
    7. Major solar eclipses visible until 2025
      • August 21, 2017 -- total, across entire U.S. (Georgia)
      • April 08, 2024 -- total, across U.S. (New England)