THE SOUTHWORTH PLANETARIUM
70 Falmouth Street      Portland, Maine 04103
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43.6667° N    70.2667° W  Altitude:  10 feet below sea level Founded
January 1970
2021-2022: CXIII
"The beginning is always today."
-Mary Wollstonecraft

THE DAILY ASTRONOMER
Wednesday, April 27, 2022
Solar Eclipse Series # 3: Visibility Region

To begin part three of our solar eclipse series, we offer the following
diagram:

[image: total-solar-eclipse-com.png]
*Umbra/penumbra:*  The umbra refers to the inner part of
the moon's shadow.  The penumbra is the lighter outer shadow
located to both the north and south of the umbra.
Image: Timeanddate.com


We see that the moon's shadow consists of two parts: the dark, inner shadow
called the *umbra* and the lighter outer shadow known as the *penumbra. *Notice
that the umbra produces a tapering shadow cone. Consequently, the resultant
umbral path shadow, called the *totality path*, will be quite narrow.*

On April 8, 2024, observers within the solar eclipse's visibility region
will either see a total solar eclipse or a partial eclipse.  Only observers
within the totality path will observe a total eclipse.  Observers within
the penumbra will observe a partial eclipse, the magnitude of which
decreases with increasing distance from the totality path.

                     [image: SE2024Apr08T.png]

The above image shows the region in which the April 2024 solar eclipse will
be visible.   The darker blue band extending from the South Pacific to the
North Atlantic represents the totality path.  Observers in this path will
see the total eclipse.  Observers located just outside this band will
observe a partial eclipse that will almost appear total.     As one moves
farther away from the band, the magnitude/obscuration of the eclipse is
reduced.

Note:  *Magnitude* refers to the percentage of the Sun's diameter the moon
covers.  *Obscuration *refers to the area of the Sun's disc the moon
covers.

Let's use an example to estimate the obscuration at a given location.
We'll choose extreme Southern California.  Observe that the magnitude line
traversing that area reads 0.6.   At that location, an observer will see
60% of the Sun's diameter covered by the moon at the moment of greatest
eclipse. Based on the chart below, we see that the obscuration will be
approximately 50 percent.  As another example, the magnitude 0.2 line runs
through British Columbia.  Someone watching the eclipse from this location
would observe 20% of the Sun's diameter blocked at the moment of greatest
eclipse. That magnitude corresponds to a 10% obscuration:  scarcely
noticeable.

[image: graph.gif]

We see that the eclipse will not be visible in Alaska or most of South
America, even though the Sun shall appear above the horizon in those
locations during the eclipse.

The red starburst along the totality path pinpoints the location of *Greatest
Eclipse,* the point at which the axis of the moon's shadow is closest to
Earth's ceneter. The other starburst labeled *sub-solar* refers to the
Sun's zenith position at the moment of greatest eclipse.

Next week, part 4: Will this be the most watched solar eclipse ever?

*However, the width of the totality path for this eclipse will measure 124
miles at maximum extent. We'll explore this issue in greater detail when we
post the article pertaining to solar eclipse extremes.

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