Favorable and Unfavorable Oppositions

For making approximate estimates, the time between oppositions may be taken as roughly 2 years and 7 weeks. It can now be seen why the apparitions of Mars occur at intervals of a little over 2 years (p. 5). It is at these times that Mars and Earth are closest and the conditions are most favorable for observations of Mars. A contributory factor is that the planet is then directly opposite from the Sun and this provides the best conditions of visibility. Because the eccentricity of the orbit of...

The Planets of the Ancients

Four thousand years ago, soothsayers, watching the skies for omens from which they might predict the future, observed that the stars circled the heavens every night from east to west. With the exception of a mere handful, however, the stars always seemed to remain in fixed positions with respect to one another. As an aid to their identification, these so-called fixed stars were later divided into groups now known as constellations (fig. 2.1). Within each of the constellations the stars formed...

The Diameter of Mars

The simplest method for determining the diameter of a planet is to measure its apparent angular diameter when the planet is at a FIGURE 4.1. Determination of the diameter of FIGURE 4.1. Determination of the diameter of known distance from Earth. In figure 4.1, for example, in which the size of the planet is greatly exaggerated for clarity, the angular diameter is indicated by 0. If D is the distance from the planet to Earth and 6 is expressed in radians (1 radian 57.296 degrees of arc), then...

Carbon Dioxide in the Martian Atmosphere

Spectroscopic studies of Mars, made over a period of more than 80 years, had produced no conclusive evidence concerning the nature of the atmospheric gases. All that could be said was that if certain gases were present, then their abundances must be less than various specified amounts. But there was no positive proof that these gases were present at all. Although it was quite certain that Mars had an atmosphere, and approximate estimates had been made of its pressure, nothing was definitely...

Planets and Asteroids

The planet mars is a member of the solar system that is to say, it is one of the bodies which revolve in orbits about the Sun. During the early years of the 17th century, when Kepler's views of a Sun-centered system were beginning to find acceptance, six planets were known. In order of increasing distance from the Sun, they were Mercury, Venus, Earth, Mars, Jupiter, and Saturn. These bodies are all visible, at appropriate times, by the unaided eye. In spite of the development of the telescope,...

The Bright Areas

The orange and yellowish-brown areas which constitute the major part about 70 percent of the surface of Mars, mainly in the northern hemisphere, are responsible for the reddish color of the planet as seen by the unaided eye. In 1809, H. Flaugergues thought that the reddish areas were large clouds, but their permanent nature indicated to others that this was not so. Later, they were regarded as dry land, by contrast with the dark areas which were considered to be bodies of water, and the...

Identification of Gases by Absorption Spectra

Sunlight passes through the Martian atmosphere, first to the planet, and then after reflection by the surface and the atmosphere. In each passage the molecules and atoms present in the atmosphere of Mars absorb radiations of characteristic wavelength, and it is by the study of these absorption spectra on Earth that information concerning the composition of the Martian atmosphere has been obtained. Of course, if a particular species does not absorb any of the radiation from sunlight or the...

Info

Photographs of Mars taken at favorable (left) and unfavorable (right) oppositions. (Lowell Observatory photographs.) The apparent diameter of Mars, as seen by the eye or, better, through a telescope, is inversely related to its distance from Earth. The ratio of the distances between the two planets at the most favorable and least favorable oppositions is roughly 35 63. The ratio of the apparent diameters of Mars is 63 35, or 1.8, so at the most favorable oppositions Mars appears to...

Martian Surface Temperatures Calculated Temperatures

An approximate idea of the maximum and average temperatures of the surface of Mars can be obtained by supposing that the planet behaves as an ideal (blackbody) emitter of radiation. The rate E at which energy is radiated from a blackbody is related to the absolute temperature T in K, which is 273 plus the temperature in C, by the theoretical (Stefan-Boltzmann) expression E 8 28 X1011 T4 cal cm7min If it is assumed that the planet has attained temperature equilibrium with its surroundings, as is...

Difficulties In Observing Mars

Mars is a highly interesting subject for study, but it is also a difficult one. Both Earth and Mars orbit about the Sun, but Earth travels at a faster speed in a smaller orbit. Consequently, the distance between Earth and Mars is always changing. At intervals of about 26 months, however, the two planets come fairly close to each other, and these occasions, which are called apparitions by astronomers, provide the opportunity for studying Mars at moderately close range. The opportunities are not...