Astronomy - USA 2021-04)

6 ASTRONOMY • APRIL 2021

ASTRO LETTERS

Universal achievement

Your recent January edition titled “The beginning

and end of the universe” was the best issue you have

ever published. Additionally, of the various science

magazines to which I subscribe, your recent edition far

surpasses anything any of the others have ever sent my

way. Magnificent is the only adjective that comes close

to doing it justice. Many thanks to all of those who par-

ticipated. — John Vaillancourt, Toronto, ON

Looking at Jupiter’s moons

Klaus Brasch and Leo Aerts, in their very nice article

about Jupiter’s moons, mention that though the moons

were discovered by Galileo, their names came from his

contemporary, Simon Marius.

Marius independently discovered the moons of Jupiter,

but one day after Galileo! Galileo published Sidereus

Nuncius to include the moons of Jupiter, but Marius

didn’t publish his results until 1614, in his Mundus

Iovialis Anno MDCIX Detectus Ope Perspicillum (Worl d

of Jupiter in the Year 1609 Detected With a Perspiculum).

Also, Marius was using the Julian calendar, so his

date of first noting the moons came out in 1609, but that

translated to one day after Galileo’s notes in January

1610 on the new Gregorian calendar that Galileo, in

Italy, was using. The discrepancy played a role in Galileo

savaging Marius, ruining his reputation for centuries.

— Jay Pasachoff, Williamstown, MA

Correction

Several of our readers pointed out a mistake in the

“Solar energy” illustration on page 45 of our November

issue, in “Gold from the stars.” That illustration

incorrectly shows the first step of the proton-proton

cycle that powers fusion in the Sun as a neutron and

proton combining to create deuterium plus a positron

and a neutrino. The correct first step of the cycle is

two protons combining to create those by-products.

The four largest
moons of Jupiter: Io,
Europa, Ganymede,
and Callisto. NASA/JPL/DLR

We welcome

your comments

at Astronomy Letters,
P.O. Box 1612,

Waukesha, WI 53187;

or email to letters@
astronomy.com.

Please include your

name, city, state, and
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and clarity.

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