نجوم و کیهان‌شناسی

حتی یک تلسکوپ بسیار کوچک هم می­تواند چهار قمر بزرگ مشتری را به­دور آن شناسایی کند. این قمرها به ترتیب فاصله از مشتری، یو، اروپا، گانیمید و کالیستو^[1] نام دارند. اندازه آنها قابل مقایسه با ماه است. کشف آنها در سال1610 توسط گالیله به وی نشان داد که تمام اجسام منظومه شمسی به­دور خورشید نمی­گردند، واین گواه دیگری بر مدل کپرنیکی منظومه شمسی بود.

[1]- Io, Europa, Ganymeade and Callisto

[2]- این دوره تناوب مربوط به حالتی است که زمین، مشتری و خورشید تقریباً در یک راستا هستند. در این نقطۀ مداری، برای چند روز، فاصلۀ زمین از مشتری تقریباً ثابت می­ماند. (مترجم)

Even a very small telescope can detect the four major moons of  Jupiter as they
weave their way around it. In order of  distance from Jupiter, they are called Io,
Europa, Ganymeade and Callisto and are comparable in size with our Moon .
 Discovered by Galileo in 1610, they showed him that Solar System
objects did not all have to orbit the Sun, giving further evidence for the  Copernican
model of  the Solar System.
Observations in 1676 made by the Danish astronomer Christensen  Roemar
of  the times of  their eclipses as they passed behind Jupiter led to the first
 determination of  the speed of  light. An eclipse of  Io occurs every 42.5 h – the
period of  its orbit – and it thus provides a form of  cosmic clock. However,  Roemar 112 Introduction to Astronomy and Cosmology
observed that the 40 orbits of  Io during the time that the Earth was moving
towards Jupiter took a total of  22 min less than when the Earth was moving away
from Jupiter ∼6 months later. The change in apparent period is due to the Dop-
pler effect and this enabled him to calculate the ratio of  the velocity of  light to
the orbital speed of  the Earth around the Sun. He derived a value for this ratio of 
about 9300. As the orbital speed of  the Earth is ∼30 km s^-1 this gave a value (actually
calculated by Christiaan Huygens from Roemar’s observations) for the speed of 
light of  about 279 000 km s^-1.