Because Venus orbits inside Earth's orbit, we see it pass through phases like the moon as the planet revolves around the sun in 225 days. When it's nearly between Earth and sun, Venus is a crescent. When it's farthest off to the side as it is today, it appears half illuminated.Â Illustration: Bob King
Venus reaches greatest elongation from the sun today. That means we see it at its maximum distance from the sun in the sky, some 47 degrees. This is equal to nearly five fists held at arm’s length. Anyone who’s gone out at dawn can attest Venus rises well before the sun and blazes like white fire in the black sky. Here in Duluth, today’s maximum separation means the planet rises at 4:08 a.m. or 3 3/4 hours before the sun.
Venus at dawn. The planet reaches greatest western elongation (maximum distance west of the sun) today.
Venus goes through phases just like the more familiar moon. That’s because it’s an inner planet. During the course of Venus’ orbit around the sun, we see the planet move from one side of its orbit (where it is today) to the farside, over to the other side and then between us and the sun. The changing geometry of the Venus-sun-Earth trio causes the degree of illumination or phase to vary during that time.
Venus looks like a half moon this week as seen in a small telescope.
Notice that Venus can never appear behind the Earth like the outer planets. Because planets like Mars and Jupiter orbit behind and beyond Earth, they occasionally appear directly opposite the sun in the sky, rising at sunset and shining due south in the midnight sky. Venus is on a leash that never exceeds 47 degrees, so it never gets far enough away from the sun to shine in the sky at midnight. Mercury, being closer to the sun, is on an even shorter leash – just 28 degrees or three fists held at arm’s length. Since both planets never stray too far from the sun, they’re most easily visible during morning and evening twilight.
Orion's Belt is the jumping off point for finding the zodiac constellation Taurus the Bull. A line through the Belt upward takes you to Aldebaran. It and the Hyades star cluster form the bull's face. The two horns extend to the left, while the Pleiades cluster marks his shoulder. This map shows the sky around 7 p.m.
Our local forecast is for clear skies and subzero temperatures tonight. If you’re willing to brave the cold after dinner, step outside around 7 o’clock and look toward the southeast. You’ll see the famous three stars of Orion’s Belt a comfortable distance above the horizon.
The mythological figure of the Bull will help you visualize the constellation as the ancients saw it.
Shoot a line upward through the Belt until you smack into the next bright star. That’s Aldebaran (al-DEB-ah-ron), the brightest star in Taurus the Bull and distinctive for its orange-red color. A small group of dimmer stars in the shape of the letter V spread out to the right of Aldebaran and form the face of the Bull. These little stars belong to a star cluster called the Hyades. Look above and to the right of the Hyades to find the most famous star cluster of all – the Pleiades or Seven Sisters. How convenient that two of sky’s brightest naked eye clusters are located near one another in sky.
Let’s return now to Aldebaran. Reach your fist out to the sky and look a little less than two fists to the left of and a little above Aldebaran to find the two stars that represent the tips of the Bull’s horns. Sky watchers in search of a modest challenge can try finding the Bull’s front legs look to the right and below the Hyades.
The association of a bull with the stars of Taurus is a very ancient one going back at least to the days of Mesopotamia circa 4000 B.C. In Babylon, it was the first constellation of the zodiac and known as the Heavenly Bull. Back then, Taurus marked the spot the sun occupied on the first day of spring or vernal equinox. Because of a slight wobbling in Earth’s axis called precession, the sun now sits in Taurus on the first day of summer. Any clear winter night, you can share this distant historical connection.