Predawn moon and Jupiter last weekend of April 2008
The Planet Jupiter
(Credit: NASA)
Friday, April 25, 2008.
Throughout this weekend, the waning gibbous moon and the blazing planet Jupiter shine close to one another in the dawn and predawn sky.
For most of planet Earth this weekend, the moon and Jupiter appear closest together on Sunday morning. Jupiter, named after the king of the gods, deserves the king planet title. This gas giant world has more than twice the mass of all the rest of the solar system – planets, moons and asteroids – combined. Jupiter’s volume exceeds that of Earth by some 1,300 times. In fact, Jupiter is like a mini-solar system within itself. It has a total of 63 known moons.
Many of Jupiter’s moons are very small. But the 4 moons originally discovered by Galileo in 1610 are large, and they’re easy to spot in a telescope or even binoculars. These moons are called the Galilean moons for Galileo, and they’re fun to watch because they change positions from night to night, as they orbit around Jupiter. The two inner moons – Io and Europa – are about the size of Earth’s moon. The two moons farther out – Ganymede and Callisto – have a diameter half again greater than our moon’s. They’re about the size of the planet Mercury.
What’s especially cool about the 3 innermost Galilean moons is that they exhibit what astronomers call an orbital resonance. Every time that Ganymede orbits Jupiter once, Europa orbits Jupiter twice and Io orbits Jupiter 4 times. This weekend, look for the moon and Jupiter to light up the early morning hours, with the king planet outshining the brightest stars.
Io, the innermost Galilean moon, revolves around Jupiter at a mean distance of 422,000 kilometers (262,000 miles), circling Jupiter in about 1.77 Earth-days. That distance compares to that of Earth’s moon, which resides an average 386,000 kilometers (240,000 miles) away. In contrast to Io’s short orbital period, the moon circles the Earth every 27.3 days.
For people familiar with Kepler’s 3rd law (orbital period squared = distance cubed), it’s possible to obtain good approximations for the distances to Jupiter’s other moons Europa and Ganymede. For example, Europa’s orbital period is twice that of the innermost Galilean moon, Io. Therefore, to find Europa’s distance:
orbital period x orbital period = distance x distance x distance
2 × 2 = distance x distance x distance
4 = distance x distance x distance
1.5874 = distance of Europa relative to Io
Since we know Io is 422,000 kilometers from Jupiter, that places Europa at about 670,000 kilometers away (1.5874 × 422,000 = 669,882.8).
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My eternal question is complexing to me. Which planet is the fifth in orbit? As a matter of fact, how does it go? Murcury-Mars-Earth-Venus-Jupiter-Saturn-Neptune? Do they ever switch?
… Sorry if this question seems dense, but you guys give official answers for the sake of arguement
Stratfort,
The planets in their outward order from the sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. No, the orbits don’t switch. However, when the dwarf planet Pluto is near perihelion (closest point to the sun in its 248-year orbit), it’s actually closer to the sun than Neptune is. Pluto was last at perihelion in 1989, and was closer to the sun than Neptune was from 1979 to 1999.
Bruce