BRIGHT PLANET ROUNDUP – July 2017
Mercury – The smallest planet is not in a very good position for viewing this month. It stays close to the horizon, rising a maximum of only about 8° on July 30th. Mercury will fade in brightness from Magnitude -1.0 on July 1st, to +0.4 on the 31st.
Venus – Look for Venus low on the eastern horizon a couple of hours before dawn (three hours by the end of July). This month its gibbous phase increases slightly in size but its disk gets slightly smaller; the combined effect decreases Venus’ brightness slightly by the end of the month. (If you observe Venus through a telescope or binoculars, be cautious of the nearby Sun!)
Mars – Mars is lost in the Sun’s glare this month. The red planet is directly in line on the far side of the Sun (at “conjunction”) on July 27th.
Saturn – Look for Saturn at its highest (due south) about midnight on July 1st, and before 10 p.m. by the end of the month. Saturn’s rings are now tilted to their maximum visibility, and although the gas giant is relatively low in the sky it’s still a great view through even a small telescope.
Jupiter – Jupiter continues to dominate the night sky, and sets about 1 a.m. during early July and about two hours earlier than that by the end of the month. Jupiter dims slightly over the course of the month, from Magnitude -2.0 to -1.9, as the surface disk decreases in size a little. That bright star to the lower left by about 11° (about the width of your fist at arm’s length) is Spica, the brightest star in the constellation Virgo; this gap will decrease to about 8° by the end of July.
SAGITTARIUS AND SCORPIUS – The Archer and The Scorpion
Around 10 or 11 p.m. this month, if you look to the south you will find the zodiac constellations of Sagittarius (the Archer) and Scorpius (the Scorpion). Adding to the magnificence of these constellations is the background of the densest portions of Milky Way – in fact, the center of our galaxy is located just west of Sagittarius’ “tea spout,” as shown below.
I don’t particularly like the tradition of assigning animals, heroes, and monsters to the constellations because most people (myself included) have trouble visualizing these ‘cartoons’ in the sky. This is especially true with Sagittarius: the “archer” (see the figure, above right) is very difficult to imagine. If we need to have an depiction for this constellation, I prefer a ‘teapot’ (see the annotated photo, below). I have to admit, though, that the traditional representation of a scorpion is a reasonably accurate one for Scorpius (above left) – it’s easy to see the curved ‘stinger’ at the base of this huge constellation.
From sky lore, Sagittarius is shooting an arrow at the scorpion’s heart, depicted in the myth by the red-giant star Antares (literally, “rival of Mars” due to its red color). According to various myths the scorpion was placed in the sky after an epic battle with the hunter Orion, who was killed in the encounter. Zeus placed both the hunted and the hunter in the sky as a tribute, but they cannot appear in the sky at the same time. Every winter Orion hunts the night sky, but hides out as summer enters the scene and Scorpius appears.
When the Moon appears as a thin crescent, you have probably noticed that most of the Moon is illuminated by a dim rust-colored light. This is called earthshine, which is sunlight reflected back to the Moon from the brightly-lit Earth. This effect has been called “the old Moon in the new Moon’s arms” and in Europe it is called “the ashen glow.”
You can actually see faint details such as the Moon’s maria (darker areas) and highlands (lighter areas) through the earthshine by using your unaided eye, binoculars or a telescope, but be advised that earthshine will illuminate features differently than direct sunlight and from a slightly different incoming angle.
Earthshine was first described by Leonardo da Vinci in the 1500s, which is quite a feat of imagination (Copernicus’ heliocentric solar system theory wasn’t published until 1543, twenty-four years after Leonardo’s death). In the Codex Leicester (written by da Vinci circa 1510), his explanation of earthshine was that the “ghostly glow,” as he called it, was caused by sunlight reflecting from Earth’s oceans – he got it right!
Why don’t we see earthshine at times other than a crescent moon? The phases of the Moon and Earth are complementary; that is, any given phase of the Moon is the inverse of the phase on Earth. For example, when the Moon’s phase is crescent the Earth is a much brighter gibbous, but as more of the Moon is lit by the sun less reflected Earth light is available to illuminate the dark areas of the Moon.
We see more or less earthshine at any given time mainly due to cloud cover at the time we are observing. For example, clouds reflect about 50% of the sunlight that hits them (known as their “albedo”), ice with snow has an albedo of 90%, and unobstructed land has an albedo of only about 10% to 25%.