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April 2005 Feature: Jupiter's Opposition & A Tale of Two Eclipses


On April 3, our planet Earth passes between the Sun and Jupiter, an event that astronomers call an opposition. At or near opposition, the Earth makes its closest approach to Jupiter for the year, and in turn, Jupiter shines most brightly in our sky. What's more, the jovial planet lights up the night sky from dusk till dawn, rising in the east around sunset and setting in the west around sunrise. Around local midnight, the King of the Planets transits the meridian and perches at its highest altitude for the night.

This giant of a planet -- some 1,300 times the volume of planet Earth -- is sometimes called the year star. That's because this planet takes some 12 years to circle through the constellations of the zodiac, staying in each constellation for roughly a year. Jupiter now shines in front of Virgo the Maiden, not too far from the constellation's brightest star, Spica. At this time next year, you'll see Jupiter in front of Libra the Scales, close to the constellation's alpha star, Zubenelgenubi. Jupiter's opposition happens about one month later with each passing year.

April, with its chorus of peepers springing into song, certainly presents a fine time for scoping out this heavenly body and its entourage of moons. A small telescope easily picks out Jupiter's four major moons: Io, Europa, Gabymede and Callisto. All four orbit with the same hemisphere facing Jupiter -- just like our Moon orbits with the same side facing our planet Earth. The first three moons are locked into an orbital resonance, with Io revolving around Jupiter 4 times for every 2 times that Europa revolves around Jupiter, and every 1 time that Ganymede does. Callisto is expected to join this resonance in the future. For lots more information on Jupiter and its many moons, click here and here.

A good amateur telescope should show you Jupiter's North and South Equatorial Belts, plus Jupiter's Great Red Spot. Remember that Jupiter rotates on its axis in about 10 hours, so the hemisphere containing the Great Red Spot may or may not be facing your way when you're looking at the King Planet. For more on Jupiter's Great Red Spot -- including optimal viewing times -- click here.

Virgo leads to Southern Treasures

The great Southern Treasures -- the Southern Cross and the Omega Centauri globular star cluster -- climb highest in our northern skies when the constellation Virgo stands on the meridian, or due south. As luck would have it, both Jupiter and the Southern Cross reach their highest points in the sky at about the same time -- or at least they will for the next few months. If you reside at 25 degrees north latitude or farther south, look below Jupiter to catch this southern signpost on or near your southern horizon. And when the star Spica crests at upper transit, look for the Omega Centauri cluster about 35 degrees beneath Virgo's sparkling blue-white star. If you live at 35 degrees N. latitude or farther south, you have a good chance of seeing Omega Centauri, perhaps the most glorious cluster in all the heavens. It is possible, however, to see this cluster from Pelee Point, Canada (42 degrees N. latitude), when seeing conditions are absolutely ideal.


April 8 Hybrid Eclipse of the Sun

A central solar eclipse is caused by the new Moon passing smack dab between the Earth and the Sun. When the Moon is close enough to Earth, the Moon completely covers over the Sun, resulting in a total solar eclipse. When the Moon is too far away, the Moon doesn't totally cover over the Sun's disk, leaving a thin annulus, or ring, of sunshine surrounding the new Moon -- an annular solar eclipse. The hybrid solar eclipse on April 8 is a rarity in that it's neither completely total nor completely annular but what's called a hybrid annular-total eclipse.

The eclipse starts off as an annular eclipse, then changes to a total eclipse, and then changes back into an annular eclipse. Click on this eclipse map to better visualize this eclipse path, which starts to the east of New Zealand, crosses the Pacific Ocean and ends in Venezuela. The annular-total eclipse path is extremely narrow, with the annular part of the eclipse colored in pink and the total part colored in blue. A much broader swath of the Earth will see a partial solar eclipse -- like the southern United States, where the eclipse happens during the afternoon hours on April 8. More information on this eclipse can be found at Fred Espenak's excellent eclipse page.

If I may digress momentarily, I wish to talk about the Moon illusion, whereby the Moon looks larger on the horizon than it does when it's high overhead. The illusion is quite an irony, since the Moon is roughly 4,000 miles closer to us when it's overhead than when it's on the horizon. This 4,000 miles, by the way, represents the radius of the Earth.

The hybrid eclipse of April 8 happens for this reason: the Moon's farther away at the beginning and the end of the eclipse than at mid eclipse. At the very beginning of the eclipse, the Moon and Sun are seen sitting on the eastern horizon at sunrise. At the middle of the eclipse, the Sun and Moon stand high on the noontime meridian. At the end of the eclipse, the Moon and Sun sit on the western horizon at sunset. (Click here for a super cool animation of this eclipse, thanks to A.T. Sinclair.)

Solar Eclipse Diagrams

April 24 Penumbral Lunar Eclipse

This penumbral lunar eclipse follows the April 8 solar eclipse by a fortnight -- the shortest possible period between a solar and a lunar eclipse. If we could put a great big movie screen at the Moon's distance from Earth, the Earth's shadow would appear like a big target in the sky, with the dark inner shadow (the umbra) looking like the bull's-eye, and the outer, fainter shadow (the penumbra) making up the outer portion of this celestial target. During a penumbral eclipse, the Moon passes through the outer, fainter penumbral shadow but totally misses the bull's-eye, or the umbra. If, indeed, a picture is worth a thousand words, I invite you to click right here.

This eclipse would be more impressive from the Moon, where you'd see the Earth partially eclipsing the Sun.

I've read a number of times that on a worldwide scale, solar eclipses are more common than lunar eclipses by about a 5 to 3 ratio. That's only if you choose to ignore penumbral lunar eclipses. When you include these eclipses -- as I believe you should -- the number of solar and lunar eclipses is virtually the same.

To compare the number of solar to lunar eclipses for the course of thousands of years, click here and here.

Lunar Eclipse Diagrams

copyright 2005 by Bruce McClure


Observing Eclipses Safely

Starry Sky Planetarium

March 2005 Feature * May 2005 Feature