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Sierra NightSky

"Let your soul stand cool and composed before a million universes" Walt Whitman

Current Moon PhaseThe Moon's current phase, courtesy of USNO.

Sierra NightSky for the period starting April 11, 2014 by Jim Kaler

The next Skylights will appear Saturday, April 19.

A busy week lies ahead (or lay behind, depending on when you read this). The Moon starts off in the waxing gibbous phase as it grows to full the night of Monday, April 14 (really the morning of Tuesday the 15th), when it will undergo a fine total eclipse that's visible throughout almost all of North America plus western South America (but not in Europe).

It then rather anticlimactically spends the remainder of the week in the waning gibbous phase, which is terminated at third quarter next week, on Monday the 21st. The evening of Sunday the 13th finds the Moon to the west of Mars . By the following night, it will have flipped to the other side.

The morning of the eclipse the darkened Moon will be in a lovely setting just to the northeast of Spica, with Mars to the west. The morning of Wednesday the 16th, the waning Moon goes just south of Saturn.

The heart of the eclipse begins at 12:58 AM CDT the morning of Tuesday, April 15, when the full umbral shadow of the Earth takes its first bite out of the leading (eastern) edge of the lunar disk. The partial phase ends when the Moon enters totality at 2:07 AM, which maximizes at 2:46 AM with the northern edge of the Moon just missing the central core of the shadow. The Moon starts to leave full shadow, when it gets the first glimpse of sunlight, at 3:25 AM, and then completely leaves the full shadow behind at 4:33 AM, not long before moonset.

Add an hour for Eastern Daylight Time, subtract one for MDT, two for PDT, three for Alaska, and five for Hawaii. Even in totality, though quite dark and red, the Moon is visible as a result of sunlight scattered and refracted by the Earth's atmosphere into the umbral shadow.

The degree of darkness depends on the state of the Earth's atmospheric blanket, particularly on recent volcanic action that makes it more opaque. Since the Moon passes just south of the central shadow, the northern limb of the Moon will be the darkest at mid-eclipse.

The penumbral stages, where the Moon is in just partial earth shadow, are not much worth bothering with, though the effect can be seen as a slight dimming just before and after the main eclipse.

In addition to the eclipse, we get to admire a fine array of planets. First up is Jupiter, which has already entered the high western sky by the time the sky is dark, and is with us until 2 AM.

By midnight, reddish Mars, making a fine color contrast with Spica to the southeast, has also crossed the southern divide. Just past opposition to the Sun, the planet's motion is obvious over only a few nights. It makes its closest approach to the Earth (0.62 Astronomical Units, 93 kilometers, 58 million miles) for this orbital round on Monday the 14th.

Next is Saturn, which rises just past the end of evening twilight and crosses to the south around 3 AM.

Finally, just before dawn, Venus rises in the east and takes over the sky, not fading away in the until bright morning twilight.

If that is not enough, Venus passes less than a degree north of Neptune on Saturday the 12th, Vesta (the brightest asteroid) and Ceres (at 470 km, 290 miles, the largest) both go through opposition to the Sun (respectively on Sunday the 13th and Tuesday the 15th), and Pluto begins retrograde motion on Monday the 14th.

From Mars and Spica, look toward the north to the brightest star in the northern hemisphere, Arcturus, the Bear Driver, who follows Ursa Major around the northern pole. As Ursa Major's Big Dipper passes nearly overhead, the Little Dipper of Ursa Minor rises up to meet it.

STAR OF THE WEEK: 5 UMI (5 UrsaeMinoris)

At fourth magnitude (4.25), number 5 in the Flamsteed list of numbered stars in Ursa Minor was obviously too faint for Bayer to make note of via Greek letter (though he and others indeed lettered fainter ones).

Five Ursae Minoris may also be among the more prominent stars that everybody with a dark sky sees but pays no attention to. It's supremely easy to find and, given its position of only 14 degrees from the North Celestial Pole, it's visible from nearly everywhere north of 14 degrees north latitude any time of night.

The front bowl stars of the Big Dipper, Merak and Dubhe (Beta and Alpha UMa), are famed as the Pointers to Polaris, the North Star. Similarly, the front bowl stars of the Little Dipper, Pherkad and Kochab (Beta and Gamma UMi), point right to 5 UMi. Only it's a lot closer, just 2 degrees north-northeast of second magnitude Kochab.

It's so close that it might make a good addition to the Little Dipper as a drop of water flying off its lip. Beyond that, the star is among the most common types in the visual bestiary, a class K (but cooler than normal, K4) giant (oddly the same as Kockab) 359 light years away (with a likely uncertainty of just 5 light years).

The recorded temperatures have a large range, from 4070 to 4395 Kelvin, with an average of 4230, which somewhat compromises correction for infrared radiation. Using the average temperature, 5 UMi shines with the light of 413 Suns (with an uncertainty of perhaps 20 percent, really not all that bad).

Temperature and luminosity then give a radius of 38 times that of the Sun, 0.18 Astronomical Units, roughly 60 percent the size of Mercury's orbit. Were it in our solar system, the orange star would glower nearly 20 degrees across, about the separation between Betelgeuse and Rigel.

A low projected equatorial rotation speed of 1.9 kilometers per second indicates a rotation period that (were the rotation axis perpendicular to the line of sight) could be as long as 2.5 years. The theory of stellar structure and evolution give a mass perhaps as high as 4 times that of the Sun and show that the star is probably fusing its core helium to carbon and oxygen, though it could be just past that state as it makes ready to become an even bigger giant.

The iron content is about three-fourths solar, nothing unusual, nor is the speed of motion relative to the Sun. At four solar masses, stars don't live very long; 5 UMi had a hydrogen-fusing lifetime of just 180 million years. After sloughing off its outer envelope, the core will turn into a white dwarf of just under 0.8 of a solar mass.

At 59 and 23 seconds of arc away are thirteenth and tenth magnitude "companions" whose motions clearly show both to be just line-of-sight "optical" coincidences. Five UMi is, however, listed as a mild barium star, which suggests the vague possiblility that there is a white dwarf companion that as a swollen enriced giant contaminated the star we see now.

Do you have a favorite star or one you would like to see highlighted on the Star of the Week? Send a suggestion to Jim Kaler

Sierra NightSky thanks to Jim Kaler.
Check out his site for more




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