Ole RÝmer

Olaus Roemer:
the Satellites of Jupiter


The First Measurement

" The first successful measurement of c was made by Olaus Roemer in 1676. He noticed that the time between the eclipses of the moons of Jupiter was less as the distance away from Earth is decreasing than when it is increasing. He correctly surmised that this is due to the varying length of time it takes for light to travel from Jupiter to Earth as the distance changes. He obtained a value equivalent to 214,000 km/s which was very approximate because planetary distances were not accurately known at that time. "
(source: Relativity FAQ by Philip Gibb, www.public.iastate.edu/~physics/sci.physics/faq/measure_c.html)
Read more: R.W. Oldford paper excerpt.



The NJ-SAS handy Events Calendar for the jovian satellites. This calendar summarizes Jupiter visibility and Galilean satellites events data. Another part of the calendar shows the satellite positions.


For historical notes on Galileo, his discovery of the Jovian world, and the naming of the Jovian satellites, see for example The Galileo Project at Rice U. (Texas). In particular the year 1610.

Visibility of Jupiter

Jupiter, as an outer planet, is least visible when it is farthest from the Earth, because it then aligns with the Sun (conjunction). It is best visible when it is closest to the Earth, because it is then in opposition to the Sun and is high in the night sky around midnight. The best observation times are the six month windows centered on solar oppositions of Jupiter: Jan 1 2002 and on Feb 2 2003. Jupiter visibility

Jupiter Satellite Events

The events include eclipses, occultations, shadow transits and body transits. Here are tables of predicted events and times. Galilean satellites phenomena

Fact sheets

Jupiter/Earth comparison fact sheet
Jovian satellites fact sheet