# ARGUMENT OF PERIFOCUS

 See ARGUMENT OF PERICENTER. argument of perig

## Related Terms

### NYQUIST STABILITY THEOREM

The theorem that the net number of counterclockwise rotations about the origin of the complex plane carried out by the value of an analytic function of a complex variable, as its argument is varied around the Nyquist contour, is equal to the number of poles of the variable in the right half-plane minus the number of zeros in the right halfplane. Also known as Nyquist stability criterion.

### ARGUMENT OF LATITUDE

The angular distance measured in the orbital plane from the ascending node to the orbiting body; the sum of the argument of pericenter and the true anomaly

### LUNISOLAR PERTURBATION

Perturbations of the orbits of artificial earth satellites due to the attractions of the sun and the moon. The most important effects are secular variations in the mean anomaly, in the right ascension of the ascending node, and in the argument of perigee.

### ARGUMENT OF PERICENTER

The angle at the center of attraction from the ascending node to the pericenter point, measured in the direction of motion of the orbiting body. Also called ARGUMENT OF PERI- FOCUS.

### ARGUMENT

One of the values used for entering a table or diagram.

### LONGITUDE OF PERICENTER

An orbital element that specifies the orientation of an orbit; it is a broken angle consisting of the angular distance in the ecliptic from the vernal equinox to the ascending node of the orbit plus the angular distance in the orbital plane from the ascend- ing node to the pericenter, i.e. the sum of the longitude of the ascending node and the argument of pericenter.

### DECLINATION DIFFERENCE

The difference between two declinations, particularly between the declination of a celestial body and the value used as an argument for entering a table.

### CRITICAL TABLE

A single entering argument table in which values of the quantity to be found are tabulated for limiting values of the entering argument. In such a table interpolation is avoided through dividing the argument into intervals so chosen that successive intervals correspond to successive values of the required quantity, called the respondent. For any value of the argument within these intervals, the respondent can be extracted from the table without interpolation. The lower and upper limits (critical values) of the argument correspond to half-way values of the respondent and, by convention, are chosen so that when the argument is equal to one of the critical values, the respondent corresponding to the preceding (upper) interval is to be used.

### EPHEMERIS TIME

The time scale used by astronomers as the tabular argument of the precise fundamental ephemerides of the sun, moon and planets. It is the independent variable in the gravitational theories of the solar system. It is determined in arrears from astronomical observations and extrapolated into the future, based on International Atomic Time.

### MERIDIAN ANGLE DIFFERENCE

The difference between two meridian angles, particularly between the meridian angle of a celestial body and the value used as an argument for entering a table. Also called HOUR ANGLE DIFFERENCE.

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