The vertical circle through the magnetic east and west points of the horizon. |

The vertical circle through the magnetic east and west points of the horizon. |

## Related Terms |

## PRIME VERTICAL CIRCLEThe vertical circle perpendicular to the principal vertical circle. The intersections of the prime vertical circle with the horizon define the east and west points of the horizon. Often shortened to PRIME VERTICAL; Sometimes called TRUE PRIME VERTICAL to distinguish from magnetic, compass, or grid prime vertical, defined as the vertical circle passing through the magnetic, compass, or grid east and west points of the horizon, respectively. |

## PRIME VERTICALA vertical circle passing through East and West points on the horizon. |

## COMPASS PRIME VERTICALThe vertical circle through the compass east and west points of the horizon |

## GRID PRIME VERTICALThe vertical circle through the grid east and west points of the horizon |

## VERTICAL CIRCLEA great circle of the celestial sphere through the zenith and nadir. Vertical circles are perpendicular to the horizon. The prime vertical circle or prime vertical passes through the east and west points of the horizon. The principal vertical circle passes through the north and south points of the horizon and coincides with the celestial meridian. |

## ZENITH DISTANCEAngular distance from the zenith; the arc of a vertical circle between the zenith and a point on the celestial sphere, measured from the zenith through 90°, for bodies above the horizon. This is the same as COALTITUDE with reference to the celestial horizon. |

## HORIZONTAL INTENSITY VARIOMETEREssentially a declination variometer with a larger, stiffer fiber than in the standard model; there is enough torsion in the fiber to cause the magnet to turn 90 out of the magnetic meridian; the magnet is aligned with the magnetic prime vertical to within 0.5 so it does not respond appreciably to changes in declination. Also known as H variometer. |

## PRINCIPAL VERTICAL CIRCLEThe vertical circle passing through the north and south celestial poles. The intersection of the principal vertical circle with the horizon defines the north and south points of the horizon. |

## SWINGING THE ARCThe process of rotating a sextant about the line of sight to the horizon to determine the foot of the vertical circle through a body being observed. Also called ROCKING THE SEXTANT. |

## CELESTIAL MERIDIANA great circle of the celestial sphere, through the celestial poles and the zenith. The expression usually refers to the upper branch, that half from pole to pole which passes through the zenith; the other half being called the lower branch. The celestial meridian coincides with the hour circle through the zenith and the vertical circle through the elevated pole. |

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