Show #38: Retrograde motion

Carpe Noctem - Seize the Night!

Image credit: NASA - The image of Sirius A and Sirius B taken by Hubble Space Telescope. The white dwarf can be seen to the lower left

CANIS MAJOR

The great Overdog,
That heavenly beast
With a star in one eye,
Gives a leap in the east.

He dances upright
All the way to the west,
And never once drops
On his forefeet to rest.

I'm a poor underdog,
But to-night I will bark
With the great Overdog
That romps through the dark.

Robert Frost, 1928

Quick News

Water identified in extrasolar planet atmosphere. (Lowell Observatory press release) Lowell Observatory astronomer Travis Barman has found strong evidence for water absorption in the atmosphere of transiting planet HD209458b The identification reported here takes advantage of the fact that HD209458b, as seen from Earth, passes directly in front of its star every three and half days. As a planet passes in front of a star, its atmosphere blocks a different amount of the starlight at different wavelengths. In particular, absorption by water in the atmosphere of a giant planet makes the planet appear larger across a specific part of the infrared spectrum compared to wavelengths in the visible spectrum.

HD 209458 b is an extrasolar planet that orbits the Sun-like star HD 209458 in the constellation Pegasus, some 150 light-years from Earth's solar system. HD 209458 is an 8th magnitude star, visible from Earth with binoculars. The radius of the planet's orbit is one eighth the radius of Mercury's orbit. This small radius results in a year that is 3.5 Earth days long and an estimated surface temperature of about 1000 degrees Celsius or around 1800 degrees Fahrenheit. Its mass is 220 times that of Earth's (0.69 Jupiter masses), which indicates that it is probably a gas giant.

HD209458b is a world well-known among planet hunters. In 1999, it became the first planet to be directly observed around a normal star outside our solar system and, a few years later, was the first exoplanet confirmed to have oxygen and carbon in its atmosphere.

Retrograde and Direct motion

Retrograde Rotations

Most planets rotate (spin on their axis) in the direct sense: they spin in the same direction as they orbit the Sun. Which is to say their north rotational pole and north orbital pole point in similar directions, more or less in the direction of the Solar north pole. If you were outside our solar system looking down the sun and most of the planets would appear to rotate counter-clockwise or anti-clockwise as you prefer. The exceptions to this rule are Venus and Uranus.

Venus is nearly always described as having its axis at 3 degrees and a rotation of -243 days, rather than 177 degrees and +243 days, in essence it is rotating normally but flipped completely upside down.

Uranus on the other hand lays on its side with its N-S axis parallel to the orbital plane instead of perpendicular. Uranus has an axial tilt of 82 degrees and a negative rotation of -17 hours, or, equivalently, of having an axis tilted at 98 degrees and a positive rotation. Since current speculation is that Uranus started off with a typical direct orientation and was knocked on its side by a large impact early in its history, it is most commonly described as having the higher axial tilt and positive rotation.

Retrograde Orbits

When we observe the sky, the Sun, Moon, and stars appear to move from east to west because of the rotation of the Earth (diurnal motion)is relatively quick, a day. This equates to the daily rising and setting of the Sun, Moon, constellations and planets. However if we study the position of the planets, relative to the background stars, over time they appear to travel, pause, reverse direction, pause, and then resume their direct, or eastward, motion around the Sun. It is this peculiar motion that baffled our astronomical ancestors and probably why the Greeks called our fellow solar system brethren 'planetes' or wanderers.

When looking inward, to Venus or Mercury, the motion we see is the direct orbit of those planets around the Sun. Their orbits are faster than ours and closer to the Sun so when we see them moving away from the Sun, pause, return toward the Sun, vanish, and then appear on the other side we observing their direct orbits around the Sun. The pausing and change of direction here are artifacts of our position in the same plane as much like sitting on the ground watching a child on a merry-go-round. The child is moving in a circular orbit around the center of the merry-go-round not shifting back and forth as the same-plane-view might delude us into thinking. This interior position allows Mercury and Venus to appear as both morning and evening 'stars' in the sky much the same way the child appears to the left or right of the center of the merry-go-round. For a real life study just observe Mercury.

On the other hand the superior planets, those with orbits outside of Earth's, have a longer orbital period. Our orbit period is faster which changes our line of sight. While we are behind in our orbits the planet we are catching up to appears to move eastward against the background stars but as we get closer to conjunction the planet appears to slowdown, stop, and turn moving westward. Once we pass conjunction and pull 'ahead' of the other planet it again appears to pause and reverse following us in a direct or eastward orbit.

Again, trotting down to the playground would be a good way to experiment. This time you need to take your place on the outside of the merry-go-round and carefully observe someone in the distance rolling beyond you in an orbit around the merry-go-round and watch their progress against background objects. But lacking a near by play ground here are some illustrations that might help.

Credit & Copyright: Tunc Tezel (Astronomy Picture of the Day - Dec 16, 2003) Mars is the bright object illustrating retrogradation and the fainter object in the background is Uranus in its direct motion.

Animation of Mars 2003 credit: NASA

For example Saturn has been slowly moving westward away from Regulus (in Leo) for the past few months and on the 20th of this month will appear to pause, turn and return on its direct (eastward) motion across the sky where as Jupiter has just begun it's retrograde motion.

Planets

Evening Planets

• Venus - Mag -3.9 in Taurus the bull is making several fantastic pairings for those of you who are looking for some great photo opps. Right now she is between the Pleiades and the Hyades.
• Saturn - Mag 0.3 in Leo has just finished its retrograde motion and is now moving in its direct (eastward) motion. Big, bright and beautiful and an easy catch in binoculars or small telescope.
• Jupiter - Mag -2.2 in Ophiuchus the serpent bearer is getting ready to start its retrograde motion appearing to move a little bit westward against the background stars.

Morning Planets

• Neptune - Mag 7.9 in Capricorn the sea goat
• Mars - Mag 1.1 in Aquarius the water bearer approaching....
• Uranus - Mag 5.9 also in Aquarius. Small telescope will be needed as you catch these three on the eastern horizon just before sunrise.
  
  
  
• Mercury - Mag -0.4 in Pisces the fish. Good morning viewing for those of you in the South much harder the further north you travel.

Constellations

Image Credit: Phobos Group website

Canis Major, Canis Minor - The greater and lesser dogs
Introduced by: Canis Major was in Ptolemy's list of 48 constellations but has been apart of astronomical lore since before the Egyptian.
Best known stars: Sirius (binary star A and B)is Greek for scorching.
The ancient Egyptians based their calendar on the heliacal rising of Sirius and devised a method of telling the time at night based on the heliacal risings of 36 stars called decan stars (one for each 10 degree segment of the 360 degree circle of the zodiac/calendar). For the Egyptians this marked the annual rising of the Nile and the 'dog' days of summer. You can see an animated illustration of Sirius' heliacal rising at the Stanford Solar Center's website (animation)

Roman myth refers to Canis Major as Custos Europae, the dog guarding Europa but who fails to prevent her abduction by Jupiter in the form of a bull.

He is also Laelaps, Actaeon's hound

More commonly Canis Major and Minor are Orion's hunting dogs, pursuing Lepus the hare or Taurus the Bull

Viewing

Binoculars - using the Free Mag 7 star atlas: Chart 9
M41 open cluster in Canis Major discovered, and documented, long before the advent of the telescope in 325 B.C. Aristotle described M41 as a cloudy spot.
100 stars into an area of about 25 light years across with several orange or red giants including the one towards the center of the cluster.
M47 open cluster in Puppis and this one gets better as you move from binos to telescopes! With binos it is a hazy blotch with a few sprinkles of bright light but with a telescope you can pick up 30 blue-white stars from 6-12th mag. The western most corner of the area around M47 is home to a reddish orange variable star KQ Puppis which sticks out as distinctly red in this blue-white group.
M46 open cluster in Puppis over 3X further away than M47; a good study in how distance 'appears' to our eyes. This cluster has a more uniform, denser cluster of faint stars. Sitting between Earth and M46 is NGC 2438 a planetary nebula that glows faintly at 10th mag. After you get settled in M46 use your averted vision to catch 2438 and then power-up to see how much detail you can see.
M93 open cluster in Puppis. Smaller than 46 and 47, 93 will show as half a dozen stars mixed in a dim glow for binoculars but a telescope will pick up 30 faint stars that seem to chain up in various curves and arcs.
M50 open cluster in Monoceros is fairly easy to find because it is all alone. M50 can be viewed as a faint hazy patch in dark skies and each 'bump' up in power and aperture reveals more starts. With an 8" scope and decent skies look for a triangle of stars that mark the center of the cluster

Telescope -
Maps 103 and 102 Taki's chart
NGC 2383 and 2384 the "Double Dog" clusters- 8.4 mag pair of OC in CMajor, due east of NGC 2287 (M41) just about 15 degrees.
Slide NW about 5 degrees to a 4.2 mag OC discovered by Caroline Herschel, C58 or NGC 2360

Challenge -
NGC 2207, 12.3 mag pair of face on spirals playing tug of war
NGC 2283 12.4 mag spiral galaxy just below alpha CMaj
NGC 2359 "The Duck" or "Thor's Helmet" emission nebula NNE part of CMaj

College Salute -
Start with NGC 2362 and open cluster around Tau CMaj just NE of Delta CMaj (Wezen, where the dog's legs join or the tail joins the body as you prefer). The cluster contains 40 members and is one of the youngest known star clusters. Now moving to the NNE corner of the cluster we are looking for the Big Dawg of the Big Dog; UW Canis Majoris (not to be confused with the UW Huskies). A mag 4.9 super giant spectroscopic binary and one of the most luminous and massive stars in our galaxy. The two stars are separated by 27 million kilometers and revolve around each other in less than four and a half days! (Herschel 400 object)

Sun

sunspots

The Moon

Lunar Phase Pro

Our beautiful lunar photos are courtesy of Frank Barrett at celestialwonders.com
I highly recommend his site for lunar phase photos. You can zoom in to his images for more detail.

Click for annotated map
Online Lunar navigation map.

Object	Latitude	Longitude	Comments
1. Plato	51.6	-9.4	(Lunar 100 and AL Lunar list)Greek philosopher c.428-c.347 B.C.
2. Valles Alpes	48.5	3.2	(Lunar 100 and AL Lunar list)
3. Cassini A/B (to the right of the number)	40.5	4.8	(AL Lunar list) Giovanni Domenico; Italian-French astronomer (1625-1712); Jacques J.; French astronomer (1677-1756)
4. The 3 Greeks (largest to smallest):Archimedes, Aristillus, Autolycus	29.7	-4	(Lunar 100 and AL Lunar list)Archimedes:Greek physicist, mathematician (c. 287-212 B.C.), Aristillus:Greek astronomer (fl. c. 280 B.C.), Autolycus of Pitane; Greek astronomer (fl. c. 310 B.C.).
5. Aristoteles w/Mitchell and Eudoxus	50.2	17.4	(Lunar 100 and AL Lunar list)Aristoteles:Greek astronomer, philosopher (383-322 B.C.), Mitchell: American astronomer (1818-1889), Eudoxus:Greek astronomer (c. 408-355 B.C.)
6. Posidonius	31.8	29.9	(Lunar 100 and AL Lunar list)Of Apamea; Greek geographer (135(?)-51(?) B.C.)
7. Serpentine Ridge (Dorsum Smirnov and Dorsum Lister)	20.3	23.8	(Lunar 100)Martin Lister; British stratigrapher, zoologist (1639-1712), Sergei Sergeevich Smirnov; Soviet Earth scientist (1895-1947
8. Lakes District			See Show #28
9. The Waterfall: Ptolemaeus, Alphonsus, Arzachel	-9.3	-1.9	(Lunar 100 and AL Lunar list)Ptolemaeus:Ptolemy, Greek astrononer, mathematician, geographer (c. A.D. 87-150, Alphonsus Alfonso X (El Sabio); Spanish astronomer (1221-1284), Al Zarkala (Arzachel); Spanish-Arabic astronomer (c. 1028-1087)
10. Rupes Recta - The Straight Wall	-22.1	-7.8	(Lunar 100 and AL Lunar list)
11. Miller, Nasireddin, Huggins	-39.3	0.8	William Allen Miller; British chemist (1817-1870), Nasir-Al-Din (Mohammed Ibn Hassan); Persian astronomer (1201-1274), Sir William Huggins; British astronomer (1824-1910)
12. Tycho	-43.4	-11.1	(Lunar 100 and AL Lunar list) Tycho Brahe; Danish astronomer (1546-1601)
AAGG Favorite: Palus Somni and Crater Proclus (coordinates)	16.1	46.8	(Lunar 100 and AL Lunar list) Marsh of Sleep, Greek mathematician, astronomer, philosopher (410-485)

Remember latitudes that are negative (-) are South and longitudes that are negative (-) are West!

Comets

Comets for the Month.

Check out the Sky Hound site.

"One touch of nature makes the whole world kin"
-- Shakespeare

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