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Discover how to take great astro-photos with your digital camera.  Capture images of the crescent moon and planets at sunset, or the star clouds of Sagittarius rising over the trees above the southeastern horizon  No experience required.  Click here to learn more…

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Eye relief is simply the distance (in millimetres) you need to hold your eye from the outer lens of an eyepiece to see its full field of view. Short eye relief means you have to jam your eye up close to the lens. This is always a problem with lower-cost eyepieces like Plossls, especially when they have short focal length.

Without glasses, 10-20 mm of eye relief is fairly comfortable. But if you need glasses at the eyepiece, look for eyepieces with at least 17-20 mm of eye relief.

Even if you don’t were glasses, your eyelashes sometimes brush against the the top lens of an eyepiece and they can leave streaks of eyelash oils that have to be cleaned off regularly. So eye relief is always a specification to keep in mind.

Fortunately, there are some good-quality long-eye-relief eyepieces on the market. The sub-$100 hardware from Orion/Skywatcher and AstroTech are worth considering, though they trade good eye relief for a modest field of view. The new Televue Delos eyepieces make no such tradeoff. You get 20 mm of eye relief with a 72 degree apparent field of view and good field flatness to the edge. The catch? These eyepieces are more than $350 each. Though they hold their value well.

If you’re new to the world of eyepieces, which are harder to choose than a telescope and just as important, here are some past articles to consider…

http://www.oneminuteastronomer.com/135/eyepiece-wisdom/

http://www.oneminuteastronomer.com/138/widefield-eyepieces/

http://www.oneminuteastronomer.com/140/planetary-eyepieces/

Where To See The Transit
The maps below shows where the 2012 transit of Venus is visible…

Worldwide visibility map for the 2012 transit of Venus (click to enlarge).

The western and central Pacific, including most of Australia, New Zealand, Fiji, and Hawaii can see the entire transit.  Western and southern Africa, Spain and Portugal, and eastern South America will not see the transit  at all because it occurs when the sun has set.  And the rest of the world can see some of the transit after the Sun rises or before it sets.

The June 5-6 transit begins at 22:09 UT (GMT) on June 5, and ends at 04:50 UT on June 6.  You can convert from GMT to your local time here: http://wwp.greenwichmeantime.com/gmt-converter/

Why It Happens
Like a solar eclipse, a transit occurs when Venus, rather than the Moon, passes between Earth and the Sun.  And like a solar eclipse, the transit requires careful alignment of the Sun, Earth, and Venus.  As seen from Earth, Venus usually passes over or under the Sun every 584 days, on average.  But the geometry and periods of the orbits of two planets cause Venus to pass in front of the Sun at well-defined intervals of 121.5 and 101.5 years in either June or December.  And the transits occur in pairs separated by eight years.  The last transit occurred on June 8, 2004.   The last pair of transits were on December 1874 and December 1882.

The Transit of Venus once held the key to understanding the size of the solar system.  In the early 18th century, Edmund Halley determined a way to measure the distance from the Earth to the Sun by precisely timing the transit of Venus from widely separated parts of the Earth.  Once this distance was known, the distances to other planets could be determined through Kepler’s Laws.

These transits were so important that most advanced nations sent astronomers around to world to measure the events of 1761 and 1769.  The transit of Venus in 1761 yielded few conclusive results despite hundreds of attempted measurements.  But the transit of 1769 was measured precisely by, among others, the team led by Lieutenant James Cook, RN, who witnessed the event from Tahiti before sailing on to claim Australia for England.  Astronomers used Cook’s measurements to calculate a distance to the Earth of 150 million kilometers, close to the now-accepted value of 149,597,870.7 kilometers.

How To See The Transit
For this June 5-6 transit, Venus will traverse the northern half of the Sun’s disk (see below)…

You’ll get the best view of the transit with a telescope, but a telescope is not required. Telescope or not, you’ll need a safe solar filter.  Here’s some advice on finding a solar filter suitable for observing this event.  If you don’t have your own filter, check if your local astronomy club is holding a public event during the transit.  They’ll have properly equipped scopes and other hardware to help you enjoy this rare event.

The transit of Venus unfolds in four stages.  First, the leading edge of the planet contacts the Sun.  Then the trailing edge makes contact, which is hard to time exactly because of the “black drop effect” that bleeds darkness from the limb of the planet as it moves onto the solar disk.  The same two stages reverse themselves as the planet leaves the solar disk.   The June 5-6, 2012 transit takes about 6 hours, which is a long time compared to the scant few minutes of a solar eclipse.  This link gives you precise timing for the four stages of the transit as seen from 121 cities throughout the world…

http://eclipse.gsfc.nasa.gov/transit/venus/city12-1.html

During the transit, the black disk of Venus, just 33x smaller than the solar disk, blocks enough light to measurably decrease the Sun’s brightness.  NASA’s Kepler observatory, in fact, uses this same idea… a transiting planet blocking light from its home star… to look for Earth-like planets around nearby stars.  Astronomers will use the 2012 transit of Venus to test new measurement techniques to find extra-solar planets using space-based telescopes.

The history and the rarity and the beauty of this event make it a compelling and memorable sight.  Please… observe it for yourself if you can.

But the idea of an expanding universe led to another shocking idea: that our universe was once smaller, perhaps at one time as small as a “primordial atom”, as Lemaitre called it, that exploded in a gigantic “Big Bang” some 13.7 billion years ago. This expansion caused the matter in the universe to cool and condense into the atoms, stars, and galaxies we see today.

The musical version of the Big Bang Theory

While familiar to most people now, the idea of the Big Bang appalled many established astronomers in the early-to-mid 20th century. And when you step back and think about it, the idea of the Big Bang seems absurd, like something from a bad nightmare of an opium-deranged 19th-century poet.

But modern astronomers and physicists buy into the idea of the Big Bang. Why? Because it best agrees with careful observation and experiment, and because it makes detailed predictions also verified by observation and experiment. Four convincing pieces of evidence suggest the universe was formed by the Big Bang:

• The Expansion of the Universe. A “Big Bang” suggests the universe “exploded” and should be expanding, which Hubble’s observations confirm. Hubble’s measurements imply an expanding universe in which galaxies don’t fly apart in fixed space, but rather space itself expands at a rate given by Hubble’s Law. The galaxies are just along for the ride.

• Cosmic Microwave Background. If the Big Bang happened, the universe must have been smaller and denser and hotter in the past, and light was jostled and scattered by the hot soup of free electrons. When things cooled down to allow electrons and protons to combine, light finally moved freely through the universe.  This first light must still be visible.  And it is… stretched to longer wavelengths in the form of microwaves which appear to come uniformly from all directions. (More about this “cosmic microwave background”, the first glow from the early universe, in future article).

• Hydrogen and Helium. In the first few minutes after the Big Bang, protons and neutrons formed out of elementary particles called quarks. Using verifiable concepts of nuclear physics, scientists predict that, if the Big Bang occurred, the early universe should contain hydrogen and helium in a ratio of 4:1 (by mass) and a few traces of lithium and deuterium (a type of heavy hydrogen). Sure enough, when astronomers look at the earliest galaxies, they observe these primordial ingredients in the right proportions.

• Galaxy Structure and Clusters.  The most distant (and therefore the earliest) galaxies appear in photographs quite different from more recently-formed galaxies. What’s more, the earliest galaxies do not form large clusters and superclusters as they do today because they hadn’t time. This is strong evidence the universe was very different in the past and that galaxies and stars must have formed suddenly about 13 billion years ago.

Some say the Big Bang is “only” a theory, and so it’s a just matter of opinion, like who will win the World Series this year. Nothing could be further from the truth. Scientists call the Big Bang a “theory” because our understanding might be incomplete, as was our understanding of gravity based on Newton’s ideas. But these four pillars of evidence overwhelmingly show that space, time, matter, and energy as we know them were created during the Big Bang.

The Big Bang theory depends on two important assumptions: physical laws are the same everywhere in the universe, and on large scales the universe looks the same from everywhere. Based on independent evidence, both assumptions seem to be correct.

But the Big Bang theory does not tell us everything. Scientists know some details of what happened just a tiny fraction of a second after the Big Bang. But it cannot yet tell us for sure where this “primordial atom” came from, or what happened before the Big Bang, or if there are other universes, or if it even makes sense to ask such questions.

By far the largest of Saturn’s moons, Titan makes up nearly 96% of the mass of all bodies orbiting Saturn, including the rings. Titan ranks as the second-largest moon in the solar system after Jupiter’s Ganymede, with twice the mass of Earths’ moon. Titan is larger than the planet Mercury.

Titan is also the only Moon with a substantial atmosphere, which is made mostly of nitrogen along with traces of methane and other organic molecules. Titan has lakes of liquid hydrocarbons and hills and dunes and boulders of frozen water (see image above). The presence of organic molecules suggests Titan may have the right chemistry for simple forms of life to develop, so planetary scientists intensely study this massive moon.

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Great reading for a cloudy night!  This new Kindle book gives you the best articles from the first four years of One-Minute Astronomer.  Tips, tales, and tours of the solar system and deep sky. Ideal for experienced and armchair stargazers.   Click here to learn more.

Saturn and its largest moon Titan as it might appear in a small telescope

For us backyard observers, Titan presents an easier target than Saturn’s rings. You can see this big moon with binoculars, while the rings are visible only in telescopes. In a larger scope, some observers can even resolve Titan into a tiny orange disk… a tantalizing glimpse of this distant, icy world.

While Titan is the biggest of Saturn’s satellites, the planet has some 62 moons with confirmed orbits. Of this large collection, nine were discovered with telescopes before the age of spaceflight. They are, in increasing distance from Saturn: Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Iapetus, and Phoebe. The moons were named after the Titans of ancient Greek legend.

With a 6″ or 8″ telescope, you can see as many as five of Saturn’s moons– Titan, Enceladus, Dione, Tethys, and Rhea. The moons move from night to night, much like Jupiter’s moons, so they are tricky to track. Sky & Telescope publishes a free Javascript utility to help you sort out which moon is which. You can find it here…

1 May.  Venus is close to its maximum brightness at magnitude -4.7.  In a telescope, the planet’s disk is a lovely crescent shape about 27% illuminated.  Today, the planet lies about 35º above the horizon at sunset.  By mid-month, it’s about 25º above the horizon, and by the end of May just  6º above the horizon, though it remains quite bright.  During this time, the planet moves closer to Earth and its crescent grows more slender but larger.

3 May.  Bright Saturn and the slightly fainter star Spica lie just east of the waxing gibbous Moon.  Saturn presents a splendid show this month.  Its rings are tilted about 14º from edge on, more than anytime in the past five years, so they are an excellent sight in a small telescope.  The planet dims from magnitude 0.3 to 0.5 and remains near the bright star Spica in Virgo for the rest of the year.

The Moon, Saturn, and Mars on May 3, 2012 at 9 p.m. local time

6 May.  Full Moon, 03:35 UTC.  The Moon is at perigee, the closest point in its orbit, and lies just 220,160 miles away.  That’s about 30,000 miles closer than at apogee.  This Full Moon will be the largest of 2012.

6 May.  The Eta Aquariid meteor shower peaks in the early morning hours.  This is usually the best meteor shower of the year for observers in the southern hemisphere, although the Full Moon will wash out the faintest meteors this year.  The radiant of this shower is at a point near the star eta Aquarii.  This shower is a close cousin of the Orionid meteor shower in October.  Both occur when the Earth passes through a debris stream from Comet Halley.

12 May.  Last Quarter Moon, 21:47 UTC.

13 May.  Jupiter is in conjunction behind the Sun and remains invisible from our skies.  By month’s end, Jupiter swings around the Sun and rises about 45 minutes before sunrise.

May 20.  New Moon, 23:47 UTC.  Today also an annular solar eclipse is visible from the coast of China and southern Japan, across the northern Pacific, and across a narrow-band through the western United States, including southern Oregon, northern California, Nevada, northern Arizona and southern Utah, New Mexico, and western Texas.  In an annular eclipse, the Moon passes in front of the Sun but is too small to entirely cover the solar disk (see image above).  This means you still need a safe solar filter to observe the eclipse.

21 May.  Look for a very thin crescent Moon just below brillant Venus in the west-northwest about half an hour after sunset.  Today Venus shines at magnitude -4.5.  The planet is 51″ across and just 8% illuminated.  For the rest of the month, try to look for the crescent shape in binoculars.  To learn more about the phases of Venus, click here…

22 May.  A thin crescent Moon lines up with Venus and the star Alnath just after sunset.  Through a telescope, you’ll see the Moon passing very close to 3rd-magnitude star zeta Tauri.  In the western U.S. and southwestern British Columbia, observers will see the Moon pass in front of this star.

The Moon and Venus just after sunset on May 22, 2012 (click to enlarge).

28 May.  First Quarter Moon, 20:16 UTC.  The planet Mars lies just northeast of the Moon.  Mars is well-placed for viewing this month, but it’s growing smaller.  It has an apparent diameter of just 10″ so use high magnification in steady sky to glimpse the north polar cap and a few dark surface features.

Compared to the ancient constellations of the northern hemisphere, Musca is a recent creation. It was one of 12 star groups created by Planicius in the late 16th century from the observations of the Dutch navigators Keyser and de Houtman. Their other constellations included exotic southern-hemisphere creatures such as the Toucan and the Peacock and the Swordfish. Perhaps their imaginations waned as they assigned this star group to the lowly fly.

The constellation Musca, the Fly, south of the Southern Cross (click to enlarge)

To confuse matters, Planicius first referred to Musca as Apis, the Bee, but this name was later assigned to a different constellation. Musca was also called Musca Australis by Nicholas Lacaille in the 18th century, since there was another constellation Musca Borealis, the Northern Fly near what’s now Aries. The unlamented Northern Fly is long gone. Only the southern fly remains.

Musca’s brightest stars are 3rd magnitude so it’s an unimpressive group compared to its northern neighbour Crux. But it holds the photogenic Dark Doodad , along with the fine globular cluster NGC 4372 at the Doodad’s southern tip. Both make for pleasant viewing in a binocular or small telescope on a cool southern autumn night. And the star beta Muscae is a fine double star separated by just 1.4″, close enough to test the vision and optics of deep-southern stargazers.

While Böotes is a large constellation, it holds just a handful of deep-sky sights for small scopes.  The region has the lovely globular M3, of course, just over the border in Canes Venatici.  But unlike neighbouring Virgo and Coma Berenices, Böotes contains few galaxies.  Indeed professional astronomers have recently mapped out what’s called the “Böotes Void”, a spherical region of space some 250 million light years across, which contains just a few dozen galaxies (see image above).

The constellation Bootes, which contains the bright star Arcturus and the lovely double star Izar. Find Arcturus by following the arc of the handle of the Big Dipper.

But Böotes does hold some fine double stars that make excellent targets for small telescopes.  My favourite is the star Izar, or ε (epsilon) Bootis.  Find Izar up the east side of the “kite” and north from Arcturus.  The star is also called Pulcherrima, which is Latin for “most beautiful”.  The star has components of magnitude 2.6 and 4.8, but they’re a close 2.9” apart.  This requires at least 100-150x to split in a 4” scope.  In nights of very unsteady seeing, the pair might be difficult to split in any telescope.

Izar’s primary is a yellow-orange giant, and the secondary is a white main sequence star.  To some observers, the primary causes the secondary to appear not white but blue or even purple.  This pair revolves around each other in 1,000 years and lies about 200 light years from Earth.

From the northern hemisphere you can see Böotes rising high in the eastern sky by 10 p.m. in April and 8 p.m. in May.  And southern observers: you can see Arcturus, Izar, and the rest of Böotes well over the northern horizon this month and next.

Publisher’s Note: An earlier version of this article suggested the globular cluster M3 lies within the boundaries of Bootes.  It lies in the constellation Canes Venatici.

First, a few tips on how to see the planet Saturn.  For the best view of Saturn, wait until it’s as high in the sky as possible before you observe so you look through as little atmosphere as possible.  Locate the planet in your finderscope, then pop a low-power eyepiece in your scope to center the planet.  At 25x, you’ll see Saturn as non-circular, and 50-60x should reveal the rings and the planet’s tiny disk.

Don’t expect a Hubble-like image.  Despite its beauty, Saturn appears quite small in a telescope.  The disk is only 20″ across, about 1/3 the apparent size of Jupiter at its closest.  The rings extend farther… about 45-50″… which makes the planet appear larger.

Now move to an eyepiece with that gives you at least 100x.  And keep going to higher magnification until the image gets too fuzzy or dim.  The optimum magnification depends on your telescope and seeing conditions.  In steady sky with a high-quality scope at 300-400x, the sight of Saturn is, in the words of one amateur astronomer, “pretty enough to make a grown man cry”.

And even nights when the air isn’t so steady, wait for moments of good seeing when the image will suddenly sharpen and jump out at you like a tiny hologram.  It’s darned impressive.

As the Voyager and Cassini spacecraft have shown, Saturn’s rings have amazingly complex structure and segmentation.  In a telescope, you’ll easily see the two main A and B rings, and in steady sky at 100x or more, you may see the large gap between the two main rings.  This is the Cassini division.

The architecture of Saturn's rings, belts, and zones as seen from Earth

Can you discern the difference in brightness between the two rings?  Most observers agree the outer ‘A’ ring is fainter than the inner ‘B’ ring.  If you have rock-steady sky and a 6-inch scope, look for the elusive Encke division, another gap near the outer edge of the A-ring.

More than most planets, Saturn displays a striking 3-D effect caused by the darkened edges of the disk and, when you can see them, the shadows cast by the rings on the planet.  The view of the rings as seen from Earth changes over Saturn’s nearly 30-year orbital period.  The rings are tilted a maximum of 27 degrees to our line of sight.  Occasionally, the rings are seen from Earth as edge-on for a few months, and they seem to disappear.

Like Jupiter, Saturn also has a complex system of cloud bands visible with a small scope.  The image above shows how they’re labelled.  These pale whitish-yellow bands are by no means as obvious as Jupiter’s, but they are visible through most scopes.  A yellow filter may help bring them out a little.