Universe Today

“What’s that bright object to the southwest at dusk?” We’ve already fielded more than a few such questions as Earth’s sister world shines in the dusk sky.  Venus just passed its maximum elongation 47 degrees east of the Sun on November 1^st, and currently shines at a brilliant magnitude -4.46. This is almost 16 times brighter than the brightest star in the sky, -1.46th magnitude Sirius.

Just like the Moon, Venus goes through a full range of phases. Through the telescope, Venus currently presents a 26.7” diameter disk. That size will swell to almost 40” by month’s end, as Venus begins to approach the Earth and presents a noticeable crescent phase. We just passed dichotomy — the theoretical point where Venus presents a half-illuminated phase as seen from Earth — on October 31^st, and Venus already shows a noticeable crescent:

Note that we say “theoretical” because there’s typically a discrepancy of a day or two between predicted and observed dichotomy. This is also known as Schröter’s Effect. One probable cause for this is the dazzling appearance of the disk of Venus. We typically use a variable polarizing filter to cut the glare of Venus down at the eyepiece.

You might also note that Venus currently occupies the “basement” of the zodiac in the constellation Sagittarius. In fact, the planet is currently as far south as it can go, sitting at a declination of -27° 14’ on this very evening. You have to go all the way back to 1930 to find a more southerly declination of Venus, just 12’ lower!

But you won’t have to wait much longer to break that record, as the chart below shows for the most southerly declinations of Venus for the next half century:

Note that each event occurs on November 6^th, and they’re spaced 8 years apart. Apparitions of Venus closely duplicate their paths in the sky over an 8 year cycle. This is because the planet nearly completes 13 orbits of the Sun for our 8. Venus “catches up” to the Earth on its interior orbit once every 584 days to reach inferior conjunction. It usually passes above or below the Sun from our vantage point, though last year it transited, a feat that won’t be witnessed again until 2117 AD.

How far south can Venus go? Well, its orbit is tilted 3.4 degrees relative to the ecliptic. It can reach a southern declination of -28 05’, though you have to go way back to 1874 for its last occurrence!

Today is also a great time to try your hand at spotting Venus in the daytime, as a 3-day old waxing crescent Moon lies about eight degrees to its upper right:

Note that seeing Venus in the daytime is surprisingly easy, once you known exactly where to look for it. Your best chances are around mid-afternoon at about 3PM local, when the daytime Moon and Venus lie highest in the southern sky. Did you know that Venus is actually intrinsically brighter per square arc second than the Moon? It’s true! The Moon actually has a very low reflective albedo of 12% — about the equivalent of fresh asphalt — while the cloud tops of Venus are more akin the fresh snow with an albedo of about 80%.

Its also worth checking out Venus and its local environs after nightfall as it passes near the Lagoon (M8) and the Trifid nebula (M8) on the night of November 6^th. Continuing with its trek across the star rich plane of the heart of the Milky Way galaxy, Venus also passes near the globular cluster M22 on November 13^th.

Venus also sits in the general of Pluto on November 15^th, lying just 6.6 degrees south of it. Be sure to wave in the general direction of NASA’s New Horizons spacecraft bound for Pluto in July 2015 tonight as well, using the Moon and Venus for a guide:

Another shot at seeing Venus paired with the Moon occurs on December 5^th.

Venus also presents a maximum area of illumination on December 6^th, and will shine at its brightest on December 10^th at magnitude -4.7. Can you catch it casting a shadow? The best time to search for this illusive phenomenon would be just before New Moon on December 2^nd. A dark sky site away from any other sources of illumination, and a snow covered ground providing high contrast also helps. Fortunately, snow isn’t in short supply in the northern hemisphere in December!

Venus is currently the only naked eye planet in the November early evening sky. We always thought that it’s a bit of a cosmic irony that the nearest planet presents a dazzling, but featureless white disk as seen from Earth. Diligent amateurs have, however, been able to tease out cloud patterns on Venus using UV filters.

Another elusive phenomenon to watch for as Venus reaches a crescent phase is ashen light. Long reported by observers, a faint glow on the night side of Venus is something that persists, but shouldn’t be. A similar effect seen on the night side of the Moon known as Earthshine is easily explained by sunlight being reflected off of the Earth… but Venus has no moon. What gives? Frequent explanations over the years have been aurorae, electrical activity, airglow, or, more frequently cited, observer bias. The brain wants to see a filled in space, and promptly inserts it betwixt the dazzling horns of the planet.

Keep an eye on Venus as it reaches maximum brilliancy and heads towards inferior conjunction on January 11^th, 2014, and a rare chance to see it on said date… more to come!