<p>It's pretty simple, really. The shadow of Jupiter, just like the shadow of the Earth, is not razor-sharp. There is an intermediate region, called penumbra, where illumination is partial. This is due mostly to the fact that the Sun is not a mere dot, but has a measurable size. The fact that the planet itself does not have a sharp edge also contributes a small amount.</p>

<p><a href="http://en.wikipedia.org/wiki/Umbra" rel="nofollow">http://en.wikipedia.org/wiki/Umbra</a></p>

<p>Penumbra can be clearly seen during Moon eclipses on Earth. The next one is on April 15 - a few days from today.</p>

<p><a href="http://eclipse.gsfc.nasa.gov/LEplot/LEplot2001/LE2014Apr15T.pdf" rel="nofollow">http://eclipse.gsfc.nasa.gov/LEplot/LEplot2001/LE2014Apr15T.pdf</a></p>

<p><a href="http://www.skyandtelescope.com/astronomy-news/observing-news/aprils-total-eclipse-of-the-moon/" rel="nofollow">http://www.skyandtelescope.com/astronomy-news/observing-news/aprils-total-eclipse-of-the-moon/</a></p>

CC BY-SA 3.0
2014-04-11T05:59:43.973Z
If you search for "jovian moons umbra" at http://scholar.google.com you'll find lot's of historical observation reports dealing with this questions. For instance: Birmingham, J. "Correspondence-Jupiter." Astronomical register 10 (1872): 94-95. http://adsabs.harvard.edu/full/1872AReg...10R..94B
Alexander Janssen
CC BY-SA 3.0
2014-04-10T17:27:30.423Z
2014-04-10T17:27:30.423Z
Florin Andrei
<p>When reading <a href="http://www.lawrencehallofscience.org/pass/passv07/jupmoons.html" rel="nofollow">an article on using Jovian moons to calculate longitude</a>, I came across this passage:</p>

<blockquote>
  <p>Periodically, Jupiter eclipses each of the four large moons as they pass into the planet's mammoth shadow. In a telescope, a moon's brightness takes several minutes to fade to black as it enters the shadow. Reappearances are just as gradual.</p>
</blockquote>

<p>How can the moons take this long to fade in and out? Is it simply the shadow creeping over the surface of the moons? What does this look like?</p>

<observational-astronomy><jupiter>
Why do Jovian moons fade when transiting Jupiter's shadow?
congusbongus
# Why do Jovian moons fade when transiting Jupiter's shadow?

(Asked by: username_0 on 2014-04-10)

When reading [an article on using Jovian moons to calculate longitude](http://www.lawrencehallofscience.org/pass/passv07/jupmoons.html), I came across this passage:

> Periodically, Jupiter eclipses each of the four large moons as they pass into the planet's mammoth shadow. In a telescope, a moon's brightness takes several minutes to fade to black as it enters the shadow. Reappearances are just as gradual.

How can the moons take this long to fade in and out? Is it simply the shadow creeping over the surface of the moons? What does this look like?

**Tags:** observational-astronomy, jupiter


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## 1 answers

### Answer by username_1 on 2014-04-10. Score: 3 | Accepted answer

It's pretty simple, really. The shadow of Jupiter, just like the shadow of the Earth, is not razor-sharp. There is an intermediate region, called penumbra, where illumination is partial. This is due mostly to the fact that the Sun is not a mere dot, but has a measurable size. The fact that the planet itself does not have a sharp edge also contributes a small amount.

<http://en.wikipedia.org/wiki/Umbra>

Penumbra can be clearly seen during Moon eclipses on Earth. The next one is on April 15 - a few days from today.

<http://eclipse.gsfc.nasa.gov/LEplot/LEplot2001/LE2014Apr15T.pdf>

<http://www.skyandtelescope.com/astronomy-news/observing-news/aprils-total-eclipse-of-the-moon/>

> **Comments:**
> **username_2**: If you search for "jovian moons umbra" at http://scholar.google.com you'll find lot's of historical observation reports dealing with this questions. For instance: Birmingham, J. "Correspondence-Jupiter." Astronomical register 10 (1872): 94-95. http://adsabs.harvard.edu/full/1872AReg...10R..94B  

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