THE SPINNING EARTH

Our planet Earth is a spherical globe which revolves around its own axis, an imagin­ary line that runs through the center of the Earth from the north pole to the south pole. If it were not for the stars beyond Earth's atmosphere, it would be difficult to prove.

A SHIFTING HORIZON As Earth rotates from west to east, our perception is that the stars, the Sun, and all other heavenly objects move around us in the opposite direction ­from east to west. The apparent motion of the stars in the sky, will depend on where you are.

At the North Pole, 90° N For this observer, the celestial north pole, marked by the star Polaris, corresponds with the zenith, the point in the sky directly overhead. From this position on Earth, the celestial equator is parallel with the horizon, and because the stars move along a parallel to this horizon, only the stars in the northern half of the celestial sphere are ever visible.

At the Equator, 0° From this latitude, an observer can see all the stars of the sky. The celestial equator rises up in an imaginary vertical line from the horizon and runs through the zenith. The north and south celestial poles lie exactly on the horizon. The stars in the east rise straight up and sink straight down again below the horizon in the west.

At 40° S Latitude For all locations lying between these latitudes, there is a part of the sky that always remains invisible, that which surrounds the celestial pole of the opposite hemisphere. The area of the sky close to the visible pole remains in view all the time. Stars here never set, but seem to circle around the celestial pole. These are called circum­polar stars, and the closer you are to the pole, the more circumpolar stars there are.

ARPA RADAR (DEFINITIONS)

DEFINITIONS OF TERMS TO BE USED with ARPA RADAR

Relative course - The direction of motion of a target related to own ship as deduced from a number of measurements of its range and bearing on the radar, expressed as an angular distance from north.

Relative speed - The speed of a target related to own ship, as deduced from a number of measurements of its range and bearing on the radar.

True course - The apparent heading of a target obtained by the vectorial combination of the target's relative motion and own ship's motion*, expressed as an angular distance from north.

True speed - The speed of a target obtained by the vectorial combination of its relative motion and own ship's motion.

Bearing - The direction of one terrestrial point from another, expressed as an angular distance from north.

Relative motion display - The position of own ship on such a display remains fixed.
True motion display - The position of own ship on such a display moves in accordance with its own motion.

Azimuth stabilization - Own ship's compass information is fed to the display so that echoes of targets on the display will not be caused to smear by changes of own ships heading.

North-up - Own ship's compass information is fed to the display so that echoes of targets on the display will not be caused to smear by changes of own sh ip' shead ing.

Head-up - Own ship's compass information is fed to the display so that echoes of targets on the display will not be caused to smear by changes of own sh ip' shead ing.

Course-up - An intended course can be set to the line connecting the centre with the top of the display.

Heading - The direction in which the bows of a vessel are pointing, expressed as an angular distance from north.

Target's predicted motion - The indication on the display of a linear extrapolation into the future of a target's motion, based on measurements of the target's range and bearing on the radar in the recent past.

Target's motion trend - An early indication of the target's predicted motion.

Radar plotting - The whole process of target detection, tracking, calculation of parameters and display of information.

Detection - The recognition of the presence of a target.

Acquisition - The selection of those targets requiring a tracking procedure and the initiation of their tracking.

Tracking - The process of obseNing the sequential changes in the position of a target, to establish its motion.

Display - The plan position presentation of ARPA data with radar data.

Manual - Relating to an activity which a radar observer performs, possibly with assistance from a machine.

Automatic - Relating to an activity which is performed wholly by a machine.