Disclaimer: links to web sites are ever-changing. It
turns out to be a Sisyphus task to keep them updated all the time.
Therefore, either try a different "spelling" of the hyperlink, look for it
on google.com and/or let me know about an outdated link by writing an e-mail
to aveh@wncc.net .
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.
About 50 people showed up for this event on the WNCC campus in Scottsbluff
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
.)
I also observed the transit on May 7, 2003 whose beginning
was visible from the Kenai in Alaska. The next transit will occur on
November 8, 2006 and will be visible from the Americas.
|
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. |
Taken around 9:20 ADT on May 6 (5:20 UT on May 7) from Soldotna on the Kenai
Peninsula in Alaska, 151W & 60N. Projection via Andy Veh's 4.5" f/4 Newton
and image with Debbie Sonberg's digital camera.
There is a big sunspot in the middle of our Sun. Mercury is the
tiny spot on the upper left. Notice how big our Sun is compared to a planet.
A link
to spaceweather's Mercury transit gallery.
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 (S&T's
Interactive Skychart)
after sunset for most of 2004.
|
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
March 2000 issue).
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.
Photo from the ESA space probe Mars
Express, January 2004. Dark sediments were probably deposited by water.
- 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!).
View of the shadow of Phobos (elliptical feature at center of each frame)
as it was cast upon western Xanthe Terra on August 26, 1999, at about 2 p.m.
local time on Mars. The image covers an area roughly 250 kilometers (155 miles)
across. The meandering Nanedi Valles is visible in the lower right corner
of the scene. Note the dark spots on three crater floors, which are probably
small fields of dark sand dunes. (c) JPL / NASA / Malin Space Science
Systems
|
The oldest Mars map, drawn by Christiaan Huygens in 1659, from "The Discovery
of Celestial Worlds", found on http://me.in-berlin.de/~jd/himmel/astro/Huygens-e.html
.
|
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 (7.16
d) 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.)
The oldest Mars map, drawn by Christiaan Huygens in 1659, from "Systema Saturnium",
found on
http://me.in-berlin.de/~jd/himmel/astro/Huygens-e.html
.
While there are many beautiful photos of Saturn, I felt that I didn't need
to reproduce any. Instead I'm showing two images that we may not be familiar
with: (1) a photo taken by an amateur astronomer (me) which resembles a view
of Saturn through a small telescope, albeit a direct view will produce a
sharper images, and (2) the diagram by the discoverer of Saturn's rings.
These two web sites give a nice overview, with drawings by Galileo, Hevelius,
Huygens, Cassini and Wren, of the history of solving Saturn's "enigma" (which
it was since nobody could have expected rings around a planet:
http://www.jpl.nasa.gov/saturn/back.html
, http://es.rice.edu/ES/humsoc/Galileo/Things/saturn.html
.
- 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
.
- Occultation of Saturn by our Moon
Photo by Koji Oshima, Japan.
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 our 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/
Calvin Hamilton's
Solar System
http://www.solarviews.com/eng/solarsys.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 ...
Find some nice animations on how extrasolar planets can be
detected at Astronomy magazine
, click on Features Stories: Science, Detecting Extrasolar Planets, then
on the
box at the bottom
(this is a quick link, if it still works). Finally choose a method.
To my WNCC Astronomy
home page
.
