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:

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.   

                     

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.  

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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