He
didn't get the credit, but he did get to name stuff...
Simon
Marius was a German astronomer and one of the earliest users of
telescopes. He discovered and named the four largest moons of
Jupiter—
Wait a minute! If you know your astronomical history, you know that it was Galileo Galilei who discovered the four largest moons of Jupiter! That's why they're called the Galilean moons!
However...
There
is this thing called “priority” in scientific discoveries. Simon
Marius claimed to have seen (in other words, discovered) the Jovian
moons before Galileo did, and he published his claim and observations
in 1614. However, Galileo had published his diagrams of the Jovian
moons four years before Marius, in 1610; Galileo pointed out that
Marius's diagram matched his earlier-published diagram. It is
possible that the two astronomers first spotted the moons on the very
same day, or that Marius copied Galileo's diagram, but most people
think that Marius discovered the moons independently but a bit later
than Galileo.
It
didn't help, when trying to figure out priority, that the two men
used different calendars. According to his notes, Simon Marius
discovered the moons on December 29, 1609; according to Galileo's
record, he discovered the moons on January 7, 1610. So Marius was
first, right?
Well,
Galileo used the Gregorian calendar, whereas Marius used the Julian
calendar. Translating Marius's date to the Gregorian calendar (which
we use today), Marius made his discovery on January 8, 1610.
Ooo,
so close!
Marius
pointed out that he looked at Jupiter a month or two before and had
seen the moons then—but he didn't write or draw anything. This is
what is known as an unrecorded observation; it doesn't count.
 |
| Callisto |
 |
| Ganymede |
So
Galileo gets the credit for the discovery. Interestingly enough,
however, we use the names that Marius gave the moons: Io, Callisto,
Ganymede, and Europa. Perhaps that's because Galileo just named the
moons I, II, III, and IV!
 |
| Europa |
 |
| Io |
Find
out more about Simon Marius in
this earlier post.
Also
on this date:
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