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diurnal parallax(geocentric parallax) The parallax of a celestial body that results from the change in position of an observational point during the Earth's daily rotation. It is the angle measured at a given celestial body between the direction to the Earth's center and that to an observer on the Earth's surface. It is thus equal to the difference between the topocentric and geocentric zenith distances, η and η0, respectively (see illustration). It is appreciable only for members of the Solar System. As the observer's position changes during the day, the diurnal parallax of the body varies from a minimum, when the body is on the observer's meridian, to a maximum, when it is on the horizon. This maximum value is the object's horizontal parallax, π. It is given by:
where r is the Earth's radius and d is the distance to the celestial object.
The Earth's nonspherical shape causes the length of the baseline to vary with latitude. If this variation is significant, as it is with nearby bodies like the Moon, then for the horizontal parallax to be a maximum it must be measured from the equator. This gives the equatorial horizontal parallax: the average value for the Moon – the lunar parallax – is 3422″.45; the mean value for the Sun – the solar parallax – is 8″.794.