Exploring the Unknown - NASA's History Office

180
OBSERVING THE EARTH FROM SPACE
Such a vehicle is one which is located at 2•01 Earth’s radii from the Earth’s centre or
about 4,000 miles from the Earth’s surface and which has a period of rotation about the
Earth of exactly 4 hours. If the Earth were not rotating the vehicle would move in the same
meridional plane through the North and South Poles. But since the Earth does rotate as
the vehicle moves, its path relative to the Earth’s surface is a series of curves.
Let us assume that at noon on March 21 the vehicle is directed poleward from the
Equator at the 95th meridian west, at “0” hour. Assuming no external perturbations, the
orbit of the vehicle is always maintained in a plane parallel to its initial orbitary plane, but
attached to the centre of the Earth in its motion through space. The Earth rotates under
the vehicle in such a way that as the vehicle proceeds northwards, it crosses all latitudes at
exactly noon and after one hour it passes over the North Pole; afterwards it then moves
southward at all latitudes at exactly midnight. At 2 hours it is at the Equator, at 3 at the
South Pole, after which it enters into the daylight hemisphere again crossing all latitudes
at exactly noon in its northward passage. At 4 hours, it crosses the [273] Equator at the
155th meridian, west, and repeats a similar path on the Earth’s surface, but displaced westward
from its initial path. In 24 hours it returns to its initial point of departure after having
made both a daylight (noon-time) and night (midnight) surveillance of the entire
Earth’s surface.
Twenty minutes after its departure on its first leg, when the vehicle has moved over
Amarillo, Texas, its horizon will enclose an area almost identical to the weather chart used
in preparation of weather forecasts for North America and adjacent oceans.
What would be seen from the vehicle at some 4,000 miles above Amarillo, Texas, at
exactly noon on June 21? An attempt has been made to portray the scene below under the
assumption that the Sun is directly overhead. In drawing a chart before sketching in the
clouds, an attempt was made to indicate the surface features of the Earth, taking into
account its normal colour and reflectivity (albedo) of sunlight, and the scattering and
depleting effects on the passage of light through the Earth’s atmosphere in the following
way:—
(a) Normal illumination values at the surface were first entered in the chart according
to zenith distance of the Sun.
(b) Next, values of the apparent illumination or “brightness” were obtained by taking
the product of the surface albedoes and the illuminations. For simplicity only two
albedo figures were used: 4 per cent. for water and 15 per cent. for land. This
then gives the brightness field of the Earth before passage of the light up through
the atmosphere.
(c) Next the Earth’s surface brightness was computed after depletion by the atmosphere,
values for which are known from the incoming sunlight.
(d) Next was computed the atmospheric contribution to the brightness field at the
vehicle. This was done by estimating from available observations, the portion of
radiation coming from the sky to the ground (i.e. the downward radiation or “skylight”)
and by assuming that the same fraction of illumination is scattered
upward. This procedure assumes that the atmosphere is a “uniform diffuse reflector”
of the brightness shown.
(e) The two brightness values—from the Earth’s surface and from the atmosphere—
are added together to give a total brightness.
To distinguish the over-all brightness contrast between ocean and land, for example,
the fractional contrast F = 2_______ BL – BO must be larger than 1/10. The computed values
BL + BO of F (not shown) are considerably larger than this value, except near the periphery, indicating
that for most of the observed area land can be readily distinguished from ocean.