Sky & Telescope - March 2019 - 15

If you could put Polaris and the
Sun side by side, Polaris would
be 2,000 times brighter.
than 10° away from true north, Earth's axis skewering the
sky nearly midway between them. During this era, the Phoenicians used the collection of stars that make up Ursa Minor
as the northward marker.
It's unclear exactly when navigators started using Polaris
specifically. In his 1953 article "The Navigation of the Norsemen," nautical historian Geoffrey Marcus mentions a telling
clue from an Anglo-Saxon poem dated circa AD 850, which
stresses the importance of the leiðarstjarna or "guiding star."
At that time, Polaris was about 7° from the north celestial
pole, and at 2nd magnitude it was only rivaled by Kocab
(Beta Ursae Minoris), which was a few degrees farther from
true north.
The leiðarstjarna appears in several later Icelandic sagas,
and in the early 1300s Dante equated the north celestial pole
with a star in his unfinished Il Convivio ("The Banquet");
in The Divine Comedy, he also describes the sky as wheeling
around the endpoint of the Little Dipper's stem. By the 15th
century, Alpha UMi fell under the moniker Stella Polaris in
European catalogs and globes. At that point, it was 3½° from

2h

0h

4h

C A S S IOP EIA

8h

6h

Polaris

C EP H EU S

Now

22h

10h

U RS A
M IN OR
2000 BC
0°

+6

20h

S&T ILLUSTR ATION

the line of Earth's rotation axis straight up from the North
Pole. The width of your thumb held at arm's length is a good
approximation for the separation between the star and the
celestial pole. But this close kinship is a temporary one. The
Sun and Moon tug gravitationally on the slight bulge around
Earth's equator, and this combined effect makes our planet's
rotation axis bob back and forth in a big circle, sweeping out
an arc through the stars that takes 25,800 years to complete.
The sweeping motion is called precession.
Back when Pharaoh Khufu was building the first Giza
pyramid in 2550 BC, it was Thuban (Alpha Draconis), not
Polaris, that shone closest to the north celestial pole. But
Earth's axis was on the move away from Draco, and some
2,000 years later both Thuban and Polaris were each more

Star magnitudes

C YGN U S
LYRA

0
1
2
3
4

Vega

DRA C O
6000 BC

12h
U R SA
MAJOR
0°
+4

10,000 BC

B OÖTE S
0°
+2

18h

H ER CU L ES

16h

Arcturus

14h

p ON THE MOVE Due to the wobble of Earth's spin axis, the north
celestial pole sweeps out a counterclockwise circle through the sky
every 25,800 years. During the development of agriculture and animal
domestication some 10,000 to 12,000 years ago, the pole was in Hercules. Right now, the pole points almost exactly at Alpha Ursae Minoris.
To find the north ecliptic pole - essentially, the north pole of the solar
system - look for NGC 6543, the Cat's Eye Nebula: Its coordinates are
nearly identical to those of the ecliptic pole.
sk yandtele scope.com * M A RCH 2 019

15



http://www.skyandtelescope.com

Sky & Telescope - March 2019

Table of Contents for the Digital Edition of Sky & Telescope - March 2019

Contents
Sky & Telescope - March 2019 - Cover1
Sky & Telescope - March 2019 - Cover2
Sky & Telescope - March 2019 - 1
Sky & Telescope - March 2019 - Contents
Sky & Telescope - March 2019 - 3
Sky & Telescope - March 2019 - 4
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