Celestial Navigation

Before the arrival of the GPS / INS era, aircraft used away from a VOR / VAR / NDB radio navigation aid network needed a navigator skilled in celestial navigation.

Daytime celestial bodies used were the Sun and Moon with the major stars, planets and the moon by night. These celestial bodies could be located by a position on the planet’s surface, called the "Substellar Point". This location can be described thus;

"The point at which a straight line joining a given star to the center of the earth cuts the surface of the earth. Thus, if one is standing at the Substellar Point of a star, it would be immediately overhead; in other words, its altitude would be 90’." [James P.160]

The Substellar Points were plotted for every instant of time through the year, with each year being different. These were published as part of the Air Almanac, a book of tables that the navigator referred to during the flight. Other tables in the Almanac included corrections for the aircraft’s height and speed. [USN Air Nav P.671 ff]

The navigator used a sextant to measure the altitude of the selected star. The altitude angle as measured from the aircraft determined the radius of a circle based on the Substellar Point. This radius was calculated from the Air Almanac.



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Since the location of the Substellar Point can also be found in the Air Almanac, a circle with this radius can be drawn, with the centre of the circle at the Substellar Point. The navigator would have already determined the assumed position of the aircraft by Dead Reckoning [D.R.], so the navigator would only draw the arc of the circle close to the assumed D.R. position.

The intersection of several celestial plots from different stars results in a navigational fix.


The Pole Star’s Substellar Point is fixed almost over the North Pole. Thus the angle from this star can be converted to produce the aircraft’s latitude, since the circle drawn from the Substellar Point located almost at the North Pole equates to a latitude reading as latitude is a circle based on the Pole. [James P.162]

The sun, moon and planets are much closer than the stars. This resulted in extra calculations to allow for the orbit of the moon around the earth and the planet’s orbits of the sun.

A highly skilled astro navigator could produce very accurate nav fixes. PG Taylor, later Sir Gordon Taylor, flying the Catalina PB2B-2 VH-ASA "Frigate Bird11", pioneered the Australian / Chile air route in 1951. At one point approaching Easter Island, the astro fix resulted in the position S26 05 W115 32 that was only 6 miles north of track. [Taylor P.250]

Taylor's Catalina VH-ASA is held today by the Powerhouse Museum, Sydney.

Later DC6 era pressurised planes could not use the perspex astrodome bubble fitted in the roof as in the DC4 / Catalina era planes, as the pressurisation would not allow the bubble window. Instead they had a special window frame built into the roof near the rear of the flight deck through which a periscope sextant was used. [DC-6B Illustrated Parts Catalog Vol 1, P. 20-432]

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Click to view full size image

Qantas used the Kollsman periscope sextant on their B707 fleet to ensure accurate navigation across the Pacific Ocean. As the B707 flew somewhat higher than the DC6, the roof fixture was different. The B707 had a double sealed periscope aperture in the cockpit ceiling.

TAA's B727 jets used this system of periscope sextants on their delivery flights from the USA, as well as for regular flights from Perth to Cocos Island until the mid 1980's.

The B747 still has this periscope sextant roof aperture but on the B747 it is called the "smoke removal hatch". [AHSA newsletter Sept 1998 & January 1999]

Page last updated on 01/01/04

Dead Reckoning
Night Beacons
Celestial Navigation