Sky and Telescope - March 2016 - 45

OBSERVING

Northern Hemisphere's Sky

Fred Schaaf welcomes your
comments at fschaaf@aol.com.

Fred Schaaf

The Power of Two
Stellar pairings offer a beauty of their own.

Castor
Pollux

Procyon
AKIRA FUJII

What can planets do that stars can't? One thing is to
move into conjunction with another celestial object.
Much of the beauty - and all of the drama - of conjunctions is that they're temporary and evolving events.
But some of the beauty is also just in the vision of two
luminaries posing close to each other in the sky. The
stars can offer us this. And let's not forget that "close" is
relative - how close together, for instance, do two 1stmagnitude stars have to be to constitute a marvel?
From planet-pairs to star-pairings. Last year was
the one I called "the Year of the Conjunctions." Several
times, pairings featured bright planets less than ½°°
degree apart when they were high in a fully dark sky. One
more such really close and readily observable conjunction
- that of Venus and Saturn - occurred in January.
An ultra-close conjunction of Venus and Jupiter takes
place this coming August, but quite low in the dusk. And
Mars and Saturn, though reaching opposition a mere 12
days apart in late May-early June, won't get closer than
7° apart in the spring. Not until August will Mars finally
have a true conjunction with Saturn, and when it does,
the two are, briefly, 4° apart. Compare that to the exactly
4° 30′ separation of a 1st-magnitude and bright 2nd-magnitude star that happen to be close in the sky for more
than a single day - in fact, for thousands of years. I'm
talking about the twin stars named for history's most
famous (albeit mythical) twins, Pollux and Castor.
The twins and the arch of spring. Mars and Saturn
are certainly brighter than Pollux and Castor this spring
and August. But that's not always so. Mars is often
dimmer than magnitude-1.58 Castor, let alone magnitude-1.16 Pollux. And Pollux and Castor's pairing is so
noticeable it has determined the identity of one of the
most prominent constellations of the zodiac.
Are Pollux and Castor really all that attentiongrabbing in a region of sky which features the brightest
constellation (Orion), the brightest star (Sirius), and the
two brightest naked-eye star clusters (the Hyades and
Pleiades)? The stellar duo is actually most eye-catching
in spring when those other sights have sunk below the
west horizon. As spring progresses, we see in the west
after nightfall what Sky & Telescope's Alan MacRobert
has called "the Arch of Spring." It's what's left after
Sirius, Orion, and Taurus have set: Procyon low in the
due west, Capella low in the northwest, and between

them, highest, at the top of the arch, Pollux and Castor.
Tighter pairings of very bright stars. Pollux and
Castor aren't bright enough to simulate perfectly a truly
dazzling conjunction of planets. But bright planetary
conjunctions are all too liable to occur near the Sun (for
one thing, two of the five classic bright planets, Venus
and Mercury, are incapable of preceding or following the
Sun by more than a few hours at most in our sky). Pollux
and Castor, on the other hand, pass high in the sky for
observers at mid-northern latitudes. And if you want
much brighter star-pairings, all you need do is head (or
already be) south.
By an amazing coincidence the three tightest pairings of very bright stars are almost equally so. At present,
Alpha (α) and Beta (β) Centauri (magnitudes -0.28 and
0.58) are 4° 25′ apart. Alpha and Beta Crucis (magnitudes
0.77 and 1.25) are 4° 15′ apart. In next month's column
we'll discuss how the rankings of the tightest bright pairs
are changing - amazingly soon. But we'll end this column with the fact that we can simulate a bright planetary
conjunction by looking at a bright double star through
the telescope - and none better than Alpha Centauri.
The brighter two of its three components are magnitudes
-0.01 and 1.35. Their separation changes rapidly and is
usually wide - but in 2016 is rather close. ✦
Sk yandTelescope.com March 2016

45


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Sky and Telescope - March 2016

Table of Contents for the Digital Edition of Sky and Telescope - March 2016

Contents
Sky and Telescope - March 2016 - Cover1
Sky and Telescope - March 2016 - Cover2
Sky and Telescope - March 2016 - 1
Sky and Telescope - March 2016 - Contents
Sky and Telescope - March 2016 - 3
Sky and Telescope - March 2016 - A
Sky and Telescope - March 2016 - B
Sky and Telescope - March 2016 - 4
Sky and Telescope - March 2016 - 5
Sky and Telescope - March 2016 - 6
Sky and Telescope - March 2016 - 7
Sky and Telescope - March 2016 - 8
Sky and Telescope - March 2016 - 9
Sky and Telescope - March 2016 - 10
Sky and Telescope - March 2016 - 11
Sky and Telescope - March 2016 - 12
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Sky and Telescope - March 2016 - 14
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Sky and Telescope - March 2016 - 16
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Sky and Telescope - March 2016 - 33
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Sky and Telescope - March 2016 - 35
Sky and Telescope - March 2016 - 36
Sky and Telescope - March 2016 - 37
Sky and Telescope - March 2016 - 38
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Sky and Telescope - March 2016 - 40
Sky and Telescope - March 2016 - 41
Sky and Telescope - March 2016 - 42
Sky and Telescope - March 2016 - 43
Sky and Telescope - March 2016 - 44
Sky and Telescope - March 2016 - 45
Sky and Telescope - March 2016 - 46
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Sky and Telescope - March 2016 - Cover3
Sky and Telescope - March 2016 - Cover4
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