Sky & Telescope - April 2022 - 45

Evenings with the Stars by Fred Schaaf
Direct from
the Dipper
A famous signpost guides you
to stellar delights.
A
pril is when the Big Dipper is positioned
highest above the horizon
in the early evening and at dusk. The
iconic asterism is so well known that it's
easy to forget that it's only the brightest
portion of the constellation Ursa Major,
the Great Bear. April is also when the
Pointers in the Big Dipper's Bowl aim
downward to Polaris, the North Star.
If you look at our Northern Hemisphere
map (pages 42 and 43), you'll
find that extending the long axis of the
Big Dipper far to the right (west) guides
you to zero-magnitude Capella, in
Auriga. April evenings are also when the
brightest winter constellations are last
visible. Taurus, Orion, and Canis Major
all drop into the Sun's afterglow, while
a few more winter groupings - Auriga,
Gemini (starring Pollux and Castor),
and Canis Minor (starring Procyon) -
hang on through May.
But what about the three brightest
stars of spring? Arcturus, Regulus, and
Spica all stand in geometric relation to
the Big Dipper, which helps us identify
them. The famous formula for locating
Arcturus and Spica is to start with the
curve of the Big Dipper's Handle and
take " the arc to Arcturus, " then drive
a " spike to Spica " (the spike being a
straight line).
Our all-sky map suggests that
proceeding south from the Big Dipper
will take you to Leo, the celestial Lion,
with its Sickle asterism that includes
Regulus (see the March issue, page 45).
In practice it's a little tricky (and hard
on your neck) to gaze directly from the
Dipper to Leo if you're at mid-northern
latitudes. That's because the zenith (the
point in the sky directly overhead) lies
between the two star groups - you
literally have to bend over backwards to
LOOKING NORTH Both the Big Dipper and Little Dipper appear in this image. During
April evenings, the Big Dipper points downwards, toward the north horizon.
reach Regulus from the Big Dipper no
matter if you're facing north or south!
The April sky map also shows that
Regulus and the Sickle are oriented
north-south, very close to the 10h line
of right ascension and, therefore, the
meridian. But if you follow the meridian
north past Polaris, you'll find
another constellation skimming the
horizon. This star pattern is the fairly
dim constellation Cepheus, the King,
which looks like a little house with a
pointy roof. You'll need clear and dark
skies to detect Cepheus when its low on
the horizon during April evenings.
Interestingly, Leo's Sickle and
Cepheus lie in exactly opposite directions,
with the Pointer Stars of the
Big Dipper roughly midway in the
vast gap between them. Those Pointers
are 1.8-magntiude Alpha (α) and
2.4-magnitude Beta (β) Ursae Majoris,
otherwise known as Dubhe and Merak,
respectively. Normally you'd expect
these to be the two brightest stars in the
constellation, but they're not. Dubhe
ranks #2 - very slightly dimmer than
Epsilon (ε) - and Merak is only #5.
Almost everyone knows that you can
find the Pole Star, Polaris, by extending
a line northward from the Pointer stars.
But do you know how to find Thuban?
Thanks to the effects of precession (the
wandering of Earth's rotational axis),
this 3.7-magnitude star in Draco, the
Dragon, closely marked the north celestial
pole around 5,000 years ago - it
was the pole star when the Great Pyramids
in Egypt were being built. To locate
Thuban, select the two brightest stars
in the Bowl of the Little Dipper, Beta
and Gamma (γ) Ursae Minoris (the
so-called Guardians of the Pole), then
run a line from them to Mizar at the
bend in the Big Dipper's Handle. You'll
find Thuban about midway between the
Guardians of the Pole and Mizar. A few
thousand years ago, that would have
been handy to know!
¢ FRED SCHAAF welcomes your letters
and comments at fcschaaf@gmail.com.
sk yand tele scope .o r g * APRIL 2022 45
AKIRA FUJII
Sky & Telescope - April 2022

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