THE EARTH AND THE MOON, OUR FAMILIAR ENVIRONMENT

Earthlight. The rising of the Earth seen from the Moon. (Apollo missions).  (JPEG, 371 K)  (NASA-JPL)

The radius of the Moon is approximately 1,700 km long, a little over one quarter of the terrestrial radius. But the moon is less dense, and therefore its superficial gravity (and therefore the weight of the objects) is approximately 6 times lower than on the Earth. The Moon has practically no atmosphere, and its surface full of craters resembles that of Mercury.  It has a lot of large dark plains, improperly called  "seas" by the first lunar cartographers, formed by very ancient basalt casts. The distance of the Moon from the Earth is 384,000 km, and  always shows the same face, while a revolution around the Earth takes 29 days. Each point on the Moon is therefore illuminated for 14.5
days, reaching a superficial temperature of 110 degrees, while, during the long lunar night, it decreases to -150 degrees.
The astronauts of the Apollo missions and the Soviet Lunik automatic probes, brought back 400 kg or lunar rocks, that revealed a maximum age for our satellite of 4.5 billion years.
 
 

Exceptional view of the Moon and the Earth together, photographed by the Galileo probe in 1992, while moving away. (JPEG, 232 K)  (NASA-JPL)

 

Picture of the Earth, taken by the Galileo probe at a distance of 2.09 million km. You can see South America in the center.  (JPEG, 329 K) (NASA-JPL)

 

It is well known that the shape of the Earth is not that of a perfect sphere. The mean surface of the oceans and the atmosphere adapt to gravitational force and to centrifugal acceleration, thus assuming a shape similar to a rotation ellipsoid, with an equatorial radius measuring 6,378 km and a polar radius measuring 6,357 km. A more detailed analysis demonstrated that the average shape of the oceans can deviate up to 100 m with respect to the shape of the rotation ellipsoid.  This is due to the presence of mass concentrations (or defects) within the Earth that create local alterations of the gravitational acceleration.

The study of the terrestrial gravitational force, along with the study of the seismic waves in connection to the large earthquakes, are the only techniques that yield quantitative data on the large scale structure within the Earth.  (JPEG, 391 K)
(NASA-JPL)

Radar picture of the volcano Vesuvius photographed from the Shuttle with the SIR-C/X-SAR instrument, in 1994. The false colours highlight the successive casts of lava.  (JPEG, 382 K)  (NASA-JPL)

 

Venice and the lagoon in a radar picture of the European satellite ERS, elaborated by I-PAF (Italian Processing and Archiving Facility), and obtained by the superimposition of three subsequent pictures in red, green, and blue. (JPEG, 632 K)  (ASI-Matera)

 

The visible side of the Moon. The craters and the hills are evident.  (JPEG, 381 K)  (Lick Observatory)

 

The western hemisphere of the Moon, that reveals to us most of the side that is not visible from the Earth. The picture was taken by the Galileo spacecraft.  (JPEG, 364 K)  (NASA-JPL)

 

The polar region of the Moon photographed from an unusual angle by the Galileo probe. The image shows very well the passage from a flat land to the zones with craters. (JPEG, 417 K)  (NASA-JPL)

 

The conquest of the Moon in 1969. Astronaut E. Aldrin carries out an experiment on the solar wind.  (JPEG, 499 K)  (NASA-JPL)

 

The famous picture of the first human footprint on an extraterrestrial body, left by astronaut Neil Armstrong on July 20th 1969. Since there are no atmospheric phenomena on the Moon, only a meteorite bombing could have erased it.  (JPEG, 692 K)  (NASA-JPL)

 

Astronaut Harrison Schmitt of the Apollo 17 mission explores the surface of the Moon, searching for geological samples. The absence of a gaseous shell that diffuses the light of the Sun is responsible for the colour of the sky: deep black.  (JPEG, 482 K)  (NASA-JPL)

 

Il GPS (Global Positioning System)

It is surprising that, in ancient times, people that belonged to great civilizations and had good astronomical culture, such as the Babylonians, thought the Earth to be flat. Homer, in 900-800 b.C., described the Earth as a disc surrounded by the river Ocean and supported by four elephants. It is thought that Pythagoras and his school (approximately 500 b.C.) were the first to speculate that if the Moon and the Sun have spherical shape, so must the Earth.

Eratosthenes, (3rd century b.C.), was the first to use a scientific method to measure the radius of the Earth. He observed that the alpha angle in the center is proportional to the distance 1 along the terrestrial circumference. In order to measure alpha, Erathostenes assumed that the radii of the Sun were parallel, and he noted that when on the city of Siene (today Assuan on the Nile) the Sun was vertical - so that it reflected in the bottom of a well), it subtended an angle alpha equal to the angle in the center in the city of  Alexandria, which is on the same  meridian,  The distance 1 between the two cities was, according to the camel drivers, was 5,000 stadias (1 stadia = 185 m). Hence the value for the terrestrial circumference of 46,300,000 meters, a value which is only 15% wrong with respect to the modern measurements.
(JPEG, 535 K)

The artificial satellites are very sensitive detectors of the terrestrial gravitational field. If the whole mass of the Earth were concentrated in one point, in the absence of perturbational effects of the atmospheric friction and of the attraction of the Moon and the Sun, the orbit of a satellite around the Earth would strictly be an ellipse (Kepler's first law). The effect of the fact that the Earth is not a perfect sphere, and that the masses are not homogeneously distributed, are  precessions and nutations of the orbital plane of the satellite which resemble the motion of a  gyroscope. These orbital perturbations are reconstructed with great precision in about thirty stations equipped with a powerful laser beam.

The instant station-satellite distance is determined by measuring the time required by a light impulse to travel on the station-satellite-station path.  Each impulse, when it is transmitted, contains approximately 1018 photons:  only a few of these manages to complete the route and be detected by the receiving system. The run time for the station-satellite-station path is a few hundredths of a second and, to be measured with the precision of the centimetre, it must be determined with an uncertainty that is not greater of the thousandth of a billionth of a second. 
By using such precision it is now possible to observe the orbital perturbations of the artificial satellites that could not be explained by the classical theory by Galileo and Newton, while they now seem compatible with Einstein's theory of General Relativity.  (JPEG, 486 e 351 K)
(University of Padua)

The terrestrial crust is a film with the mean thickness of 35 km, and represents the most superficial layer of our planet. The thickness of the crust decreases to values smaller than 10 km underneath the oceans, and doubles  near the large mountains ranges (over 80 km in Himalaya). The crust is subdivided into continental  plates.  The demarcation lines between two continental plates is represented by oceanic fractures (for example the great oceanic ridge), or by collisional ranges (for example the Andes, the Himalaya, the Alps).

The plates interact: some tend to move away from each other, other tend to overlap, others to slither horizontally one on top of the other. A great seismic activity has been observed along the lines of the major interactions.
Since the last few years the artificial satellites are used to systematically measure the variations of the relative positions of satellite stations placed  in tectonically active lines. If one assumes that such relative position variations on the surface are proportional to the elastic deformation degree of the deep rocks, it is theoretically possible to predict when the rocks will reach a compression and tension load beyond which they will disintegrate. Many earthquakes correspond to the overcoming of the rupture load of the deep rocks.  (JPEG, 602 K)
(NOAA)

To date, over 50 orbiting satellites are dedicated to the observation of the Earth. Optical images are taken at a high altitude (36,000 km) per meteorological purposes, and at a low altitude for detailed environmental studies (morphology of the terrestrial surface and its evolution, estimates of the  world scale agricultural productions, monitoring for defense purposes). Very precise navigation signals are irradiated by the 24 Global Positioning System (GPS) satellites of the United States, orbiting at an altitude of 20,000 km. The GPS satellites provide the instant coordinates and the speed of marine, terrestrial and air users. (JPEG, 321 K)  (University of Padua)

The components of the GPS receiving system (Global Positioning System) are: an antenna placed on a tripod, covered by a plastic cover, able to receive signals from satellites up to 15 degrees of            elevation on the horizon; a data acquisition terminal able to calculate immediately the geographical coordinates of the antenna. 

A practical application of great interest of GPS satellites is represented by the possibility to determine in real time the position of lorries and vans for the transport of valuables, in order to know the position of the vehicle in case of theft. The public transport companies of some cities (also Italian), are already prepared to inform passengers on the actual position of the bus and the presumed waiting time.  Each bus is equipped with a GPS satellite receiver.

Many ships, sports craft included, already use the GPS satellites for the navigation.  GPS satellites will probably be employed for the instrumental landing of airplanes.
(JPEG, 262 K)
(University of Padua)

The University of Padua is in the forefront for the scientific and civilian applications of GPS. It was the first in Italy to realize, back in 1994, a completely automatic GPS station with data quality and quantity standards weekly verified and approved by NASA. The antenna is placed on the geodesy tower of the Bo palace. The obtained data are daily used by scientists from all over the world for the calculus of the orbits of satellites, for the control of variations of the length of the day, often linked to meteorological reasons, for analyses of coordinate variations connected not only to phenomena of continental drift, but also to local deformations.

The Padua GPS station offers a support to topographers for the execution of polygonation technical surveys, calculus of areas and volumes, for the plain-altimetric description of vast portions of the territory.  (JPEG, 663 K)
(University of Padua)

The problem of the precise coordinate determination of the Galileo National Telescope at the Canary Islands was solved by the use of a GPS receiver. Approximately one hour of automatic data acquisition was enough to determine latitude, longitude and altitude of the TNG with an error of a few metres. The determination of the same coordinates by using the classical  triangulation  on the fundamental stars required two nights of hard work of two researchers, with a result that was about five times less precise than with GPS.  (JPEG, 577 K)  (University of Padua)

 
 

The Tethered Satellite System.

The TSS consists of a spherical satellite (built in Italy), anchored to the Space Shuttle through a thin conducting cable, with a diameter of 2.5 mm. With the two missions of July 1992 and February 1996, the interaction of the system with the terrestrial ionosphere for the production of electricity and propulsion in space, was verified. The studies for the application of the TSS system to the future International Space Station are being carried out at present. In 1992 was the flight of the first Italian astronaut, Franco Malerba,  who contributed to the experiment.

Artistic vision of the TSS anchored to the Shuttle.  (JPEG, 517 K)  (ASI)

 

The satellite at the Kennedy Space Center. The diameter of TSS measures 1.6 m, and its mass is 518 kg.  (JPEG, 582 K)  (ASI)

 

A phase of the preparation of TSS at Alenia Spazio of Turin that, for its realization, guided a group of National Industries among which were Laben, Proel Tecnologie, Fiar, Carlo Gavazzi, Officine Galileo, BPD Difesa e Spazio, SSI, Piaggio.  (JPEG, 655 K)  (ASI)

 

The satellite, after the detachment from the long release trellis, moves away in space.  (JPEG, 100 K)  (ASI)

 

The TSS and the Moon. Italian astronauts Maurizio Cheli and Umberto Guidoni participated to the second mission.   (JPEG, 146 K)  (ASI)

 
 

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