Recap Astronomical Observations
What can be observed and measured through the optical
Why is a shirt red ? Why is it black ? Why is the sky blue ?
(explain in terms of reflection/refraction)
What is the origin of the black lines in a star’s light spectrum ?
Explain the meaning of a red-shifted star spectrum.
What is an interferometer ?
Describe non-optical astronomy.
Describe fixed, movable radiotelescopes.
Describe satellite-based detectors.
Describe gravitational detectors.
Earth and the Terrestrial Planets
The moving sky
Earth structure and atmosphere
Our Moon and the Earth-Moon system
The Moving Sky
Earth movement: spin(24h), revolution(365.2422d)
The 23.4o tilt/declination and the seasons
Motion of the Moon (27.32d)
Longitude and time
Calendars: Julian(45 BC), Gregorian (1582 AD)
Planets orbits and speed: Mercury 48km/s, Earth
29.8 km.s, Pluto 5km/s.
Comets: high eccentricity and variable speed
Earth is more massive than Mars or Venus, has 70% of
surface covered with water. Density of material doubles as
one goes to its deeper layers.
The outer crust has 10-50km (5-10km in ocean beds)
A mantle of denser rock has 2,900km
The outer core (iron, nickel) is liquid and has 2,200km
The inner core is solid and has 1,300km. Temperature
over 5000o fueled by radioactive disintegrations.
Geophysicists determine all this from the study of seismic
Theory of continental drift (1924)
Crust+outer mantle(lithosphere) which is rigid and
Eight large (plus many small) plates flow very slowly on
top of the inner mantle
At boundaries magma rises and creates new crust
structures (ex. mid-Atlantic ridge). In addition
earthquakes and chains of volcanoes.
Some short-lived volcanoes appeared in hot spots in the
middle of a plate (ex. Hawaii islands).
Earth surface rocks vary in age corresponding to a very
dynamic history. Water and the wind also shape them.
Composition: nitrogen (78%), oxygen(21%)
Oxygen is highly reactive and must be replenished –
photosynthesis uses CO2 to produce oxygen
CO2 (0.03%) is mainly in limestone and ocean.
Like water, CO2 contributes to the greenhouse
effect, which keeps the temperature at an average of
17o. Without the atmosphere Earth’s surface
temperature would be –13o. Burning of fossil fuels
raises the temperature by 0.5o per century.
To 10km troposphere - temperature 17o to –53o
10-50km stratosphere – temperature raises to -3o
because ozone (O3) absorbs UV light.
50-80km mesosphere – temperature drops to –68o
80-150km thermosphere/ionosphere – temperature
raises to 800-2000o because of atomic and molecular
processes with UV and X rays; ions and electrons
influence radio communications
To 70,000km magnetosphere – Van Allen belts
deflect the solar wind and cause auroras at the poles.
Our Moon (I)
With its radius about a quarter of the Earth radius, our
Moon is the largest relative to the parent except Pluto’s
Elliptical motion with an average distance of 1.2 light-
seconds (or 384,400km)
Surface with cratered highlands (85%) and “seas”(15%).
Both craters and seas were formed by meteor impact. The
dark-colored seas correspond to magma spilled after
Water ice is mixed with rocks at the two poles
Most likely our Moon was formed through an impact
between Earth and a similar size body. The same impact
might be the origin of the tilt in the Earth axis.
Our Moon (II)
Our Moon Structure
No atmosphere (too small to keep it)
1-20m lunar soil (pulverized rocks)
Below the soil about 65km of lunar crust
Below the crust about 1,400km of a mantle of
The lower part of the mantle (at about 800km)
is partly molten. Minor seismic events are
generated at that level.
The center might contain an iron-rich core
(350km in radius).
The Earth-Moon System
Why do we see various lunar shapes ? It is related to
the relative positions of Earth, Moon and Sun.
Ocean tides: As a result Earth’s spin slows down
and the lunar rotation is accelerated. Moon’s spin
also slowed down and now the fact that its spin is
synchronized with its revolution results in the fact
that the same hemisphere is shown to Earth.
a solar eclipse is max.7.5 mins.
Lunar eclipses are less frequent than solar eclipses
but are seen from the whole hemisphere.
Max number of eclipses per year is seven.
Phases of Our Moon
full Moon Earth
Moon phases are related to the shape of the lighted part,
as seen from the observation point on Earth.
The terrestrial planets - Mercury, Venus, Earth
and Mars - are relatively small, dense with similar
composition and with solid surfaces.
The jovian (giant) planets - Jupiter, Saturn,
Uranus and Neptune - are much larger than Earth.
Their structure is either a hydrogen-helium gas, or
a solid icy structure.
Pluto is a small planet made out of rock and ice.
The Terrestrial Planets: Mercury
Radius=2,439km, revolution=88d, spin=59d.
70% iron and 30% rock, but small magnetic field implying
that most iron is solid. Wrinkles suggest past magma flow;
Almost no atmosphere.
Day temperature up to 425oC,
Night temperature -173oC
But its “day”=59 Earth days
Observations: Mariner 10 (1974).
The Terrestrial Planets: Venus
Radius=6,052km, revolution=225d, spin=243d.
Structure similar to Earth. Volcanic activity older than on Earth.
Atmosphere contains 96.5% carbon dioxide and 3.5% nitrogen
with clouds of sulphuric acid between 5-70km altitude.
The greenhouse effect maintains a constant 480oC.
It also makes Venus brighter than stars.
Water and carbon dioxide history different from Earth because
of distance to the Sun.
Observations: radar-Mariner 2 (1962), landed - Venera 7 (1970)
9,10 (1975), radar - Pioneer 12 (1978), Magellan (1994)
The Terrestrial Planets: Mars
Radius=3,200km, revolution=687d, spin=24h37m, same axis
tilt as Earth.
Mars is the least dense of the terrestrial planets; core contains
iron and iron sulfide; no magnetic field. Schiaparelli’s polar
caps and canals (1877), dusty red soil (iron oxides), Olympus
Mons volcano 25km altitude, Mariner valleys 8km depth,
dyed-up river beds.
Although the composition of the atmosphere is similar to
Venus, its density is about half of Earth’s atmosphere and
therefore there is no significant greenhouse effect; day 20oC
and night -70oC; ice clouds, dust storms.
Possibility of past life (Viking 1976, Antarctica meteorite
1996). Other missions: Mariner 4 (1965), Mars Pathfinder
- Dusty redish soil
(the “red” planet)
- The most
similar to Earth
Phobos (R=13.5km) and Deimos (R=7.5km).
They are captured asteroids.