Conditions for Eclipses

  • The moon must block the sun in your location. In other words, you must be in the moon's umbra or total shadow:
(Image from Twenty-First Century Astronomy by Kay, et. al.)

Types of Eclipses


  • Full moon is in Earth's umbra (total) or penumbra (partial).
  • Can be seen by anyone who can see the moon.


  • Earth is in the new moon's umbra (total), penumbra (partial), or antumbra (annular).
  • Moon's umbra on Earth's surface is narrow or does not reach Earth at all.
(Illustration from The Under-Standing of Eclipses by Guy Ottewell)

Why Isn't There an Eclipse Every Month?

  • The moon's orbit is tilted about 5 degrees relative to Earth's orbital plane (the ecliptic), so Earth, Sun and Moon are not all in the same plane (so shadows don't align) except twice per month, when the moon is near a node in its orbit. The moon must be new and near a node for a solar eclipse:
(Image from Twenty-First Century Astronomy by Kay, et. al.)
  • In addition, the line of nodes (the intersection of the moon's and Earth's orbits) precesses once every 19 years, so eclipse seasons drift. Note that January is on the right and December is on the left:
(Image courtesy of The McGraw-Hill Companies, Inc.)
A beaded-curtain calendar display (Image from The Under-Standing of Eclipses by Guy Ottewell)

Ideas/Terms Associated With Eclipses

  • Timings for total eclipses:
    • First Contact – When moon first begins to cover the sun.
    • Second Contact – Start of totality; when moon first fully covers the sun.
    • Third Contact – End of totality; when moon begins to uncover the sun.
    • Fourth Contact – When moon ends covering the sun.
  • Photosphere – The bright, visible "surface" of the sun.
  • Corona – Hot, ionized gas being "blown" out from the sun. Can be visible up to several solar diameters during totality.
  • Chromosphere – Glowing (pink) hydrogen gas just above the photosphere and visible just after second contact and just before Third Contact
  • Prominences – Glowing (pink) hydrogen gas emerging from the surface of the sun and (sometimes) returning to the surface.
  • Sunspots – Cooler regions of the photosphere that look dark, by contrast, with the rest of the Photosphere.

Photos from the Annular Eclipse in Delaware, Ohio on May 10, 1994

Observers on the Hamilton-Williams Campus Center patio. Note the projection through the refracting telescope. (Photo by Barbara Andereck)
Partial phases and maximum coverage. Eclipse was central (but still annular) in northern Ohio. (Photos by Barbara Andereck)
Compare the size of the moon relative to the sun for an annular (left) versus total (right) eclipse. (Photos by Barbara Andereck)

Saros Cycle

The conditions around the location of the sun and moon relative to Earth and the line of nodes determine the nature of a solar eclipse and the length of totality. Remarkably, these conditions repeat within a very short time frame every 18 years, 11 1/3 days. The similar eclipses that occur at this 18-year interval are part of the same Saros Cycle. The eclipse durations and paths are similar. Consecutive eclipses in the same Saros Cycle occur 120° apart, with each eclipse occur west of the previous one.

Below are five key time periods that match to produce the Saros Cycle of similar eclipses. Note how closely the first four time periods line up. The synodic month, which is new moon to new moon, is the time that determines precisely when the eclipses will repeat: 6585.321 days = 18 years 11 1/3 days

  1. 223 synodic (phase) months = 6585.321 days
  2. 242 draconic (node) months = 6585.357 days
  3. 239 anomalistic (perigee to perigee) months = 6585.537 days
  4. 19 eclipse years = 6585.781 days
  5. 18 anomalistic (perihelion) years = 6574.673 days

Below are maps showing eclipses in the same Saros Cycle:

(Illustration courtesy of Pearson Education, Inc., publishing as Addison Wesley)
(Illustration courtesy of Sky & Telescope)
(Image courtesy of The McGraw-Hill Companies, Inc.)

Antikythera Mechanism

A remarkable instrument from about 200 BCE, known as the Antikythera Mechanism, after the area where it was discovered by Greek divers in the early 20th century, tracks dozens of celestial and terrestrial cycles, including the Saros Cycle. (You may know the name from Indiana Jones and the Dial of Destiny. The mechanism has no magical powers, though its construction at such an early time is extraordinary.) One of the toothed gears in the mechanism has 223 teeth, the number of synodic months in one Saros repeat. To read more about the mechanism and its reconstruction, read this Scientific American article.

Department Contact Info


Schimmel/Conrades Science Center
Delaware, OH

Department Contacts

Chair: Brad Trees
Professor of Physics & Astronomy
Schimmel/Conrades Science Center 139
740-368-3779 |

Academic Assistant: Joshua Seiders
Schimmel/Conrades Science Center 201
740-368-3907 |