• Gem - Gemini the Twins

    IAU Constellation


    30 of 88
    Astronomical Regions

    Intro and Visual description

    Look for the two bright stars, Castor and Pollux, which form one vertex of the Winter Hexagon. Castor is closer to Capella, in Auriga on the north; and Pollus is closer to Procyon, in Canis Minor on the south. Cancer and Leo lie to the east.

    The Geminids meteor shower occurs around October 19.

  • Hya - Hydra the Water Snake

    IAU Constellation


    1 of 88
    Astronomical Regions

    Intro and Visual description

    Hydra the Water Snake is the largest and longest of the constellations, stretching from Cancer to Libra. Several constellations and asterisms ride on its back; from head to tail they are Sextans the Sextant, Crater the Cup, Corvus the Crow, and Noctua the Owl.

  • Winter Hexagon

    Asterism Visual Appearance

    The "Winter Hexagon" is a giant six-cornered pattern that is prominent in the night skies of winter. Make this hexagon pattern your frame of reference for cool autumn mornings and brisk winter evenings! For a diagram, see Astronomy Picture of the Day for 2011 Jan 3.

    The winter hexagon includes six constellations, and some of the brightest of stars visible at any time of the year from northern latitudes:

    Start with Aldebaran in Taurus, pass down to Rigel in Orion, and continue on down to Sirius in Canis Major. Then trace upward to Procyon, in Canis Minor the Little Dog. Continue on to Pollux and Castor, the two stars of Gemini, and on past them to the top of the hexagon, bright yellow Capella, lying almost straight overhead, in the constellation Auriga the Charioteer. Auriga looks more like a pentagon than a Chariot, perched on top of the horns of Taurus.

    Let's review, proceeding clockwise from Capella:

    1. Capella, the she-goat, is a bright yellow star almost directly overhead throughout the winter. It forms the top vertex of the winter hexagon.
    2. Aldebaran, the red eye of Taurus, gleaming out of the night as the bull charges down upon Orion. Look for the Hyades, a V-shaped cluster of stars that forms the bull's head.
    3. To the upper right of Sirius is Rigel, a bluish-white star, and the left foot of Orion. Look nearby for Orion's belt and bright Betelgeuse, his reddish right shoulder.
    4. Sirius, the brightest star in the sky.
    5. Located above left of Sirius, Canis Minor consists of only two bright stars, and Procyon is by far the brightest.
    6. The remaining vertext of the winter hexagon is comprised of the two bright stars of Gemini: Pollux (on the Procyon side) and Castor (on the Capella side).

    As described in Starstruck Tonight:

    The Winter Hexagon contains an unrivalled collection of stars:

    • Sirius, below, is the brightest star in the night sky.
    • Capella, above, is the 6th brightest.
    • Rigel is the 7th.
    • Procyon the 8th.
    • Betelgeuse the 10th.
    • Aldebaran, Pollux, and Castor are also among the nightÍs 25 brightest stars.
    Sharp is the night, but stars with frost alive
    Leap off the rim of Earth across the dome.
    It is a night to make the heavens our home...
    George Meredith, Winter Heavens

    The two bright stars Castor and Pollux together form one vertex of the Winter Hexagon. To the Greeks, Castor and Pollux were the twin sons of Zeus and the mortal woman Leda. Homer's Iliad tells how the beauty of their sister Helen "launched a thousand ships" in the Trojan war. With the oath "By Jiminy," sailors revered the Gemini twins as the Protectors of ships. Castor, on the Capella side, is actually six stars in one, ceaselessly revolving around one another in an intricately-choreographed cosmic dance....

  • Bode (1801), plate 2: Libra Planisphere


    Uranographia Tab II. Stellatum Hemisphaeri um Librae

    Bode included two planisphere plates. They are not southern and northern hemispheres; each one has Polaris at the top and the south pole at the bottom. Each one is centered upon an equinox point (where the ecliptic or path of the Sun and the celestial equator intersect). The March equinox point was in Aries in antiquity; by Bode’s time, due to the precession of the equinoxes, it had shifted to Pisces. The September equinox point was in Libra in antiquity; by Bode’s time it had shifted to Virgo.  Bode titled the plates as the Aries and Libra planispheres.

    The Libra planisphere, centered on the September equinox in Virgo, includes these constellations, among others, which appear high overhead in the night skies of spring:

    Equatorial:  Ophiuchus, Serpens, Libra, Virgo, Crater, Corvis, Hydra, Sextans, Leo, Cancer, Monoceros.

    Northern:  Hercules, Quadrans Muralis, Bootes, Canes Venatici, Ursa Major, Telescopium Herschelii, Gemini, Lynx, Ursa Minor.

    Southern:  Scorpius, Tubus Astronomicus, Lupus, Centaurus, Apis, Chameleon, Crux, Argo Navis, Robur Caroli II, Circinus (sector compass), Canis Major, Pixis Nautica (magnetic compass), Machina Pneumatica (air pump), Officina Typographica (printing press).

    In September, the Libra-Virgo equinox (the center of the Libra plate) is traveling with the Sun, rising in the east in the morning and setting in the west in the evening.  Imagine the center of the planisphere has the Sun pinned to it for that day, and that’s how it would move across the sky. Therefore the constellations near the center of this planisphere are invisible in the daytime sky at that time unless there is a solar eclipse.  They would be visible directly opposite the Sun at the March equinox.