Astronomy - Peculiar Planets
comparison between planets
- planet
web sites
This lecture is called "peculiar planets" because I like you to learn
a few interesting facts about the planets. I want you to look at
the planets "normal" facts as well in your textbook, such as their geology,
atmosphere, orbital and physical parameters. But these are easy to
forget. Peculiarities, though, may stick with you.
Mercury
-
Mercury's very long day
Mercury needs 58 2/3
Earth-days to rotate once on its axis while it takes it 88.0 Earth-days
to orbit our Sun once. Multiply 58 2/3
with 3 and 88.0 with 2 and you come up with the same number: 176 E-days.
That's the length of one day (and one night) on Mercury.
-
Sun reversing its course, e.g. rising, setting, and finally rising
 You're
on Mercury's surface at 90 degrees longitude looking East (at other longitudes
you would see the Sun backtrack high in the sky or set, rise, set). Frames
are 24 hours apart, so the entire process lasts several weeks.
If you stay at this location, you see another Sunrise
in 24 weeks.
Mercury's slow rotation (equivalent to 58 2/3
E-days), its highly elliptical orbit (44 million miles at
aphelion and 29 million miles at perihelion) and therefore its faster
movement at aphelion (remember Kepler 2) produce this effect. (As
you see, Mercury's "seasons" would in fact depend on distance, and also
on the long days and long nights (88 E-days each).)
-
Mercury's surface
Not everything is mapped due to lack of (satellite) data. It has no
atmosphere, wherefore it is cratered like our Moon, since no erosion is
going on.
-
Observing
Since Mercury's orbit is smaller than Earth's, the planet is always
close to the Sun and therefore only observable at dawn or dusk.
-
Mercury Transit
On November 15, 1999, Mercury transited our Sun's edge. It's
doing this about once a decade; and just like with eclipses, it depends
on if you are in the right spot. It doesn't transit at each inferior
conjunction (Mercury exactly between Sun and Earth, happens
about every four months) because Mercury's orbit is tilted by 7
degrees with our orbit; thus it usually orbits above or below our Sun.
The two nodes are always in the same place so that we can see a transit
only in May or in November. The next transit will be on May 7, 2003
(visible
from Australia, Asia, Europe and Africa; lasting 5 hours, all observable
from most of Asia, Eastern Europe and Eastern Africa).
About 50 people showed up for this event and most were amazed by the
fact that a planet is so small and our Sun is so huge. I had set
up the refractor shown below and my orange Newtonian for projecting our
Sun's image on a white screen. I used the C-5 for visual observations
and the C-8 at f/20 (with 40 mm eyepiece) for photographing the event.
(For
descriptions of these telescopes check my
telescope script.)
|
Projecting our Sun's image with a f/15 refractor.
The finder is covered. |
Projection: Sunspots on the right. Mercury is at
upper left. |
Using a proper solar filter on the C-5. All three
photos (c) Mark Rein. |
Through C-8. |
Transit between 2 and 3 pm on November 15, 1999.
East Parking lot at WNCC, Scottsbluff, Nebraska. |
All three photos (c) Andreas Veh. Our Sun's edge
should be sharp, sorry for the fuzziness. |
Venus
-
Sun rising in West, setting in East
Venus is the only planet in our solar system that rotates in the opposite
direction as it orbits. Therefore the sun rises in the West.
-
 Thick
atmosphere, Weather, Greenhouse effect
Venus has two "continents". It has a thick atmosphere. Therefore impact
craters have been eroded long since.
Its weather is terrible: mostly containing carbon dioxide, precipitating
(sulfuric) acid rain, and 100 times Earth's pressure. CO2
produces the Greenhouse effect and a sweltering 900 F.
Venus is usually totally cloudy.
Orbiting spacecraft (American Mariner) used radar to map its surface
through the thick atmosphere. Measurements of the atmosphere and surface
were made by the Russian Venera probes and weather balloons.
The Venera landers did survive for about an hour: then circuits melted
and the body was eaten by the acid and crushed at the same time.
-
always facing Earth with the same side on closest approach
Every one and a half years Venus is exactly between Earth and Sun,
i.e. closest to Earth. It faces Earth always with the same side on these
occasions.
-
Observing and phases
(In the photo on the right, Mars is marked on top,
Venus is the bright one on the bottom.) Like Mercury, Venus
orbits "inside" and therefore appears close to the sun, but is higher above
the horizon and much brighter, therefore it has the nicknames "evening"
and "morning star" (it's a planet though) - you'll see it above the Western
horizon after sunset for most of 1997.
 |
Like our Moon, Venus features phases, which can be seen in a telescope
and maybe even binoculars.
I took the upper photo on February 22, 2001, with a Pentax
mounted behind an 8 inch Cassegrain (see my telescope page), on a moonless,
clear night without wind. With a 17 mm eyepiece this made for an
effective focal length of about 15,000 mm [= 50 feet]. Due to this
large magnification, just pushing in the cable release makes the telescope
vibrate. I achieved this image by putting a black cardboard in front
of the telescope, removing it when the vibration stopped after about ten
seconds, then exposing for five seconds [thanks go to Lee Hartley for the
advice]. Since Venus was fairly low in the sky, Earth's atmosphere
and the lenses in the eyepiece caused some refraction, thus the fake orange-yellow
and blue. -- See also the Jupiter and Saturn images further down which
were taken in the same fashion. All four photos are to scale but
of course Venus appears as large as Jupiter only because it's twelve times
closer. -- Just a month later I took the lower photo dates on March
16, 2001. Venus was by then only two weeks away from inferior conjunction
(the point for Mercury or Venus that's between Earth and Sun) and consequently
bigger and showing an even more sliver like appearance. -- Since the C-8
switches left and right, I switched the Venus images back to their correct
orientation, having its lit-up side directed toward our Sun that had set
in the West. |
-
Venus transit
Venus' orbit is tilted by 3.5 degrees to ours. Since we are aligned
with Venus at one of its nodes every 13 years (in late May, early June
or December), that would produce 8 conjunctions per century, and in fact
there are 11 conjunctions this century. But for 9 of them, Venus
is behind our Sun. That leaves two transits which will come up soon:
on June 8, 2004 (visible from Australia, Asia, Europe and
Africa; lasting 6 hours, all observable from Eastern Europe and Eastern
Africa) and on June 5-6, 2012 (visible from North
America, Pacific Ocean, Australia, Asia; lasting 6 hours, all observable
from far Eastern Asia, NE Australia and Hawaii). Better watch
these events because we can't wait until December 11, 2117.
|
Earth
-
Analemma of Sun's apparent motion across the sky (also called the "equation
of tim")
 Dennis
di Cicco's incredible photograph of the Analemma (for
a description of how he did it, check out S&T's
Backyard Astronomy, it's the very last paragraph).
This is the figure 8 or infinity-like symbol on an old globe, where
it defines the time equation. See, if you find this in one of the
planet web sites.
Due to the tilt of the Earth's axis, the Sun appears at different heights
during the year. Also, again because of the axis tilt, the Sun is
"slow" near the solstices. It's "fast" near the equinoxes.
Since the Earth's orbit is elliptical (like that of all other planets),
it orbits faster when closer (Sun trails - remember Kepler 2) and slower
when farther (Sun advances), which makes the upper loop smaller and narrower.
Redshift's
Analemma, Explanation
of Analemma. Click here for the "Analemma"
home page.
Variation
in Time of Sunrise.
-
Eclipses
 Solar
Eclipse in the Caribbean, February 26, 1998. (c) Jamalee and Dan
Clark, Scottsbluff, NE.
Go to the Very Important
Topics.
-
Our Moon's phases
Go to the Very Important
Topics.
-
Moon always facing Earth with the same side
Tidal forces not only account for the rise and fall of ocean waters,
but the Earth has slowed our Moon's rotation so much that it is now synchronized
with its orbit.
-
Seasons
Go to the Very Important
Topics.
|
Mars
- Retrograde
motion (all planets)
The farther away a planet is from the Sun, the slower it orbits. Therefore
an inner planet keeps passing any planets further away: Venus passes all
planets from Earth to Pluto; Earth passes all planets from Mars to Pluto.
This has the effect that an outer planet seemingly moves "backwards".
- Polar caps, canals, Mt. Olympus
The polar caps are known to be frozen carbon dioxide (dry ice). During
the Martian summer - on the respective hemisphere, that is - the polar
caps partially melt. It is believed that water ice is locked beneath them,
which during warmer climates may have been running water producing the
Martian canals.
Olympus Mons is an extinct volcano and the largest mountain in the
solar system: 78,000 feet high (2½ times taller than Mt. Everest)
and covers an area of 70,000 square miles (almost the size of Nebraska).
Why is it that high? Because Mars' surface gravity is a factor 2½
smaller than Earth's, Olympus Mons is relatively light for its size.
The Mars crust can therefore support a much higher mountain.
Its reddish color is due to oxidized iron: rust.
- Phobos and Deimos
The two Martian moons are very small (the size of Scottsbluff county)
and have odd shapes. Since Mars orbits relatively close to the asteroid
belt, Phobos and Deimos were probably asteroids and at some time captured
by Mars.
Besides Venus, Mars is the only other planet, spacecraft (the American
Viking
probe) have landed on.
Phobos is covered by a 3 feet thick layer of dust, it's "hip
deep" (great photo!).
Asteroids
Subject: Asteroid
XF 11 animation
Subject: Spacecraft
NEAR Shoemaker near Asteroid Eros
>Since Eros is not round, I imagine maintaining an orbit
>would be quite a feat, but probably made more managable by
>the slow speed required to orbit such a relatively small
>mass.
One can show that at large enough distances the gravitational force
can be seen as emanating from the asteroid's center of mass. NEAR's
orbit is thus not endangered. But they just have to monitor carefully
its orbit because any deviations from a true Kepler orbit (closed ellipse)
give clues about Eros' mass distributions.
Asteroid Eros' density: 2.4 grams/cm^3; mass: about 5 x 10^15 kg.
NEAR's speed is amazing now: 3 feet per second, that's like strolling
down main street. Trajectory.
Subject: NASA's
Asteroid pages, fact
sheet
Close ups of asteroids
(all to scale)
 433
Eros (NEAR Shoemaker; was in orbit around Eros from 2-14-2000 to 2-12-2001).
Eastern hemisphere on top, Western below. NEAR is the first spacecraft
that landed on an asteroid, February 12, 2001. |
 253
Mathilde (NEAR Shoemaker fly-by; June 27, 1997). |
 951
Gaspra (Galileo fly-by; October 29, 1991). |
 243
Ida and its moon Dactyl (Galileo fly-by; August 28 1993).
An asteroid with a moon was an extraordinary discovery. Using Kepler
3, Ida's mass 100 x 10^15 kg is probably the most accurately determined
of all asteroids. |
Jupiter
- Cloud bands and Great Red Spot
Jupiter
has an extensive cloud system (ammonia ice, water ice, hydrosulfides),
whose bands are circling the planet. On a clear, calm night, these cloud
bands can be seen in a fairly good amateur telescope.
(For photo description see similar Venus photo.)
The Great Red Spot is a storm, appearing like a hurricane. It's twice
as big as Earth and has existed for at least 300 years, i.e. since its
discovery.
- Observations of Galilean moons
So named after Galileo Galilei, their discoverer. Io, Europa,
Ganymede, and Callisto are the largest and brightest (they reflect the
most light from the Sun) among Jupiter's 16 moons. They can be observed
even in binoculars. If tracked during continuous nights, it is easy to
notice that they orbit Jupiter - since that is what a moon is supposed
to do.
Io (1.77d orbital period), Europa (3.55d)
and Ganymede (7.16d) are also in resonance
orbits, one taking twice as long as the inner one. Their positions
on Fabio Marino's
web site.
- Volcanoes on Io (courtesy NASA - JPL, Voyager)
Again
tidal forces: Not only important for ocean tides and our Moon's synchronous
rotation and orbit, but Jupiter as well puts such strain on its closest
moon Io that it rips the surface open and heats and melts the interior,
producing volcanic eruptions (Voyager 2; eruptions at upper left; the
sky and Io should be black (its night side), but white background is better
to print).
Volcanoes and lava fields are known on other bodies in the solar
system (Venus, Mars, our Moon), but Io is the only one known to have active
volcanoes.
- Shoemaker-Levy 9
1999
hologram stamp by the German Postal Services.
The ninth comet detected by Carolyn and Eugene Shoemakers and David Levy,
one of hundreds of comets detected by amateurs each year. But the event
of a lifetime!
Comet Shoemaker-Levy 9 was captured by Jupiter's gravity, was ripped
apart after one orbit (remember tidal forces) and plunged into Jupiter's
atmosphere. The stamp shows the scenario in infrared: an impact (lower
left) and an approaching fragment. The fragment actually moves when
looking at the real hologram stamp..
Saturn
- Rings
The
most likely theory: eons ago another moon was orbiting Saturn. It was the
planet's closest moon and kept orbiting closer until it reached the Roche
limit, where the entire moon was ripped apart by ... - ... tidal forces.
The debris now makes up the rings, which are not solid, but contain myriads
of rocks and ice. (For photo description see similar
Venus photo.)
- Edge-on
Saturn showed a spectacle of its own in 1995 and 1996. Every 14 years
its rings become several times "invisible" for a week each time, simply
because we view them edge-on or sunshine is reflected edge-on.
- Janus and Epimetheus
These two moons orbit very close to Saturn (one orbit lasts 17 hours)
and to each other. The inner moon of the two orbits just a little faster
(remember Kepler 3), so that it keeps receding more and more from the outer,
eventually catching up with the outer after 4 years again (i.e. about 2100
orbits for one moon, one orbits more for the other). Then - due to their
weak, but mutual gravitational attraction - they change places (just before
they meet)! Now the outer becomes the inner moon and vice versa - and the
game starts again. You find a great animation at the Swinburne
Centre for Astrophysics & Supercomputing in Hawthorn, Australia.
Uranus
- Tilt of axis and extreme seasons
Uranus' axis is tilted by about 90 degrees. If Uranus were habitable
(which it isn't) and had seasons, standing at the North pole you would
see the Sun continuously above the horizon for 42 Earth-years (half of
Uranus' orbit).
- Discovery
The German-British astronomer William Herschel discovered Uranus in
1781, the first planetary discovery since the ancient times. He was charting
the skies - as he used to do every night - when he saw an unfamiliar point
of light. After several months he noticed that it was moving: A planet.
He wanted to name it King George's planet - the same George who fought
the Americans during the Independence war.
- Nomenclature
This doesn't really have to do with astronomy but may be of
peculiar interest as well: contrary to all other planets which derive their
nomenclature for moons and surface features from mythology (and
names of historic and more recent scientists e.g. on our Moon),
Uranus' moons and their surface features are named after characters and
places from William Shakespeare's plays and Alexander Pope's poems and
essays. E.g. Puck, Oberon, and Titania are characters from A Midsummer
Night's Dream (Ariel and Miranda are from The Tempest,
while Umbriel is from The Rape of the Lock) and are also
the names for some of Uranus' moons. For a very good overview check
Jennifer
Blue's web site at the U.S. Geological Survey.
- Observing
Uranus is on the brink of naked eye observability, but is fairly
easy to find with hand held binoculars. Once pointed at the correct
region (it helps when Uranus is in a faint Zodiac constellation such as
Cap, Aqu, Pis, Aries until about the year 2020), one compares to the supplied
chart (see my lab K1). The bright one should be Uranus.
Also, look at it a week later and notice that Uranus has moved.
Neptune
- Discovery
One of the greatest accomplishments of astronomy and mathematics was
the discovery of Neptune: Uranus' orbit showed irregularities, which could
only be explained by another planet - with its gravitational pull disturbing
Uranus.
The British Adams and the French Leverrier computed this hypothetical
planet's position and orbit in 1845 and 1846 - and the Germans Galle and
d'Areste discovered it the same year at exactly the predicted position.
- Great blue spot
Astronomy
books had to be revised in 1989 when Voyager 2 took pictures of a circular
storm (similar to Jupiter's GRS) the size of Earth.
They have to be revised again: In 1995 the Hubble Space Telescope revealed
that the great blue spot had vanished.
- Observing
Neptune cannot be observed with the naked eye; one needs a small telescope.
Find the correct region with the help of the supplied
chart (see my lab K1). Look at it a week (or a few days) later
and notice that Neptune has moved.
Pluto
- Discovery
Since Uranus still showed perturbations (that couldn't be explained
by Neptune alone), yet another planet was thought to exist. Percival
Lowell in Flagstaff, AZ, (the same who "thought to have seen" man-built
canals on Mars) started a search for Planet X. Clyde Tombaugh used an exhausting
method of flip-flopping photos of the same region of sky some days apart.
Stars would stay put while a planet would shift. In 1930, after one year
of hard work, he stumbled onto Planet X near its predicted position. He
named it Pluto (god of the underworld) and honored Lowell, who had died
in 1916, by choosing his initials.
- Planet or not
It was found that Pluto's mass was far too little to have caused Uranus'
perturbations, which later turned out to arise from miscalculations (please
don't forget that ALL calculations were done by hand - an endeavor like
calculating a planet's orbit deserves a lot of respect). Pluto does not
fit in either category of terrestrial or Jovian planets. Little is known
about Pluto's
surface, maybe it's just a huge snowball, the closest object of the
Kuiper belt. However, it's been considered a planet for already 60 years
and most likely will remain to be considered a planet.
- Pluto's moon Charon
In 1978, Christy discovered that Pluto has a moon, which was named
Charon (the guy who brings people over the river Styx). Ground-based telescopes
simply show a hump. It's hardly possible to achieve better resolution because
of atmospheric distortions. The web site The
Nine Planets shows how much better Hubble's imaging is (also
on mine). Even though there is bad resolution, a nice thing happened
for 5 years shortly after Charon's discovery: Pluto and Charon kept eclipsing
each other while orbiting each other within 6.4 days, showing up as broad
dips in their combined light curves, which allowed to determine their respective
diameters. Kepler's 3rd law permitted to compute their combined mass.
An estimate for density gave their individual masses. Recently HST was
able to resolve Pluto's surface somewhat.
Because of tidal forces, Pluto and Charon always face each other with
the same side (synchronous
orbit).
Furthermore, they are almost considered a double planet, since they
are closer in mass than any other planet-moon(s), and since their common
center of gravity is outside of Pluto.
- Pluto's eccentric orbit
Because of its eccentric orbit, Pluto crosses Neptune's orbit. Pluto
was closer to the Sun from 1979 until 1999.
Observations of planets: need to check dates, time of the night, expected
brightness, position (in Zodiac)
- naked eye: Mercury, Venus, Mars, Jupiter, Saturn
- binoculars and small telescopes: size of disk, moons, above &
Uranus, Neptune
- telescope with aperture of at least 12 inches: above & Pluto
Comparisons between Terrestrial and Jovian planets
|
..................... Terrestrial planets ..................... |
..................... Jovian planets ..................... |
| |
Mercury, ... |
Jupiter, ... |
| Composition |
rocky |
gaseous |
| Size |
|
|
| Mass |
small |
large |
| Density |
|
|
| Rotation |
|
|
| Distance |
|
|
| Period |
|
|
| Atmosphere |
thin or none |
very thick |
| # Moons |
|
|
| Rings |
|
|
|
|
|
* Pluto is in a league of its own and does not belong to either group.
Planet Lectures on the internet:
The Nine Planets
http://www.seds.org/billa/tnp/
LANL - Views of the
Solar System http://bang.lanl.gov/solarsys/homepage.htm
JPL - NASA -
Welcome to the Planets http://pds.jpl.nasa.gov/planets/welcome.htm
Extra Solar Planets
Go to Geoffrey
Marcy's home page. He and Paul Butler are the world record holders
for the most planets discovered with more than 20. Here he
explains how it's done. They are investigating only yellow Main
Sequence stars. Wonder why ...
