The dust which reaches the Earth , or is collected by spacecraft as micrometeorites makes its presence known in another way. The particles that make up this dust travel in elliptical orbits around the Sun and are concentrated in the plane of the ecliptic. They reflect the light of the Sun to give a luminous band around the sky. As seen from the Earth, the brightness of the light falls away rapidly with increasing angular distance from the Sun and the ecliptic to give what is known as the ZODIACAL LIGHT . This phenomenon has been known since antiquity and was described as the false dawn in the Rubaiyat of Omar Khayyam, the Persian astronomer-poet, around AD 1100. Because of the tilt of the Earth’s axis, the zodiacal light is most easily seen after sunset in spring and before sunrise in autumn in the northern hemisphere, and vice versa in the southern. It is best seen on a clear moonless night somewhere well away from the glare of city lights. A casual observer is often convinced that it is twilight that he is seeing because of the brightness and large extent of the light. The term zodiacal light is generally used just for the light seen within about 90° of the Sun. In 1854 T.J.Brorsen discovered a patch of light in the night sky exactly opposite the Sun; he named it the GEGENSCHEIN, which in English means reflection or counterglow. Later Brorsen also discovered a light bridge or ZODIACAL BAND joining the gegenschein to the true zodiacal light and thereby completing a band of INTERPLANETARY LIGHT along the whole ecliptic. The relative brightness of the various parts of the interplanetary light that aw visible to the naked eye. P.J.van Rhijn and others have shown that substantial amounts of interplanetary light are visible in all parts of the sky. Away from the ecliptic, the contrast is very low and as a result the light is not noticed by visual observers.

Studies of the- interplanetary light are somewhat hindered by the other sources of light in the night sky. As well as scattered moonlight and more local effects such as city lights and aurorae there are two other contributions to the undisturbed light of the night sky, the nightglow and integrated starlight. Together with the zodiacal light these three sources provide roughly equal amounts of light. The NIGHTGLOW is emitted by atoms and mole-exiles in the Earth’s atmosphere at an altitude of about 80km. Because this is relatively low, an observer on the ground sees minimum brightness directly overhead and an increase towards the horizon. INTEGRATED STARLIGHT is the combined effect of faint stars that cannot be seen individually by the naked eye. Because of the very flattened shape of the Galaxy this light is largely confined to the region of the galactic equator. To study the interplanetary light, nightglow and integrated starlight, astronomers must first separate their relative contributions. The nightglow is the easiest to remove because it is well known how it varies with altitude and it does not move around the sky as the Earth rotates. The zodiacal light moves relative to the stars as the Earth orbits the Sun and this motion allows its separation from the integrated starlight. The uncertainties inherent in these methods mean that it is not unusual for studies to differ by a factor of two or more. Satellite observations can be made above the Earth’s atmosphere in order to eliminate the nightglow. Spacecraft are being sent further and further, from the Earth, and when observations from beyond Saturn become available in the 1980s it is expected that these will show only a small contribution from the interplanetary light and will allow more accurate studies of the integrated starlight. Astronomers hope that in due course these new techniques will allow them to distinguish more accurately the three phenomena and learn more about each.

Comments are closed.