## P

: a mathematical term for the conic section obtained by cutting a cone with a plane parallel to its generator (or “edge”). A **parabolic** surface, obtained by rotating a parabola about its own axis, is much used for mirrors as it has the property of focusing parallel rays of light to a point focus.

**parallactic angle**

**[?
]** the angle of the polar triangle between the directions to the pole and to the zenith at the celestial object. Hence it is the angle between the vertical and the hour circle, of use in calculating the effects of astronomical refraction.

**parallax**: the effect whereby the apparent position or direction of an object changes with the observation

point. See solar parallax for its affect on solar parameters. The effect can affect the accuracy of reading

scales.

**paschal moon**

The first full moon following the Spring equinox. Important for the determination of Easter.

**patina**

Coloured, metallic compounds (usually oxides and sulphides) which form on metal surfaces left exposed to the atmosphere. The actual colour depends principally on the metal, but also on the impurities in the atmosphere resulting from pollution or proximity to the sea. Typically, copper-containing alloys develop a greenish colour.

**pedestal**

The supporting structure for a dial, particularly horizontals. Usually of stone, it may comprise several different pieces and brings the dial to a convenient viewing position. See Appendix VII for more details of architectural terms.

**pelorus**

An instrument for finding the solar azimuth, consisting of a magnetic compass and an alidade, with some means (e.g. mirrors, prisms, shades) of viewing the Sun and the compass needle.

**penumbra**

The area of partial shadow surrounding the central umbra. It is due to the finite size of the sun. An observer standing in the penumbra would observe only part of the sun’s disk.

**perigee**

Orbit when it comes closest to the Earth.

**perihelion**

The point in the Earth’s orbit when it comes closest to the sun. It occurs during the first week of January.

**perpetual calendar**

A device, usually in the form of a circular plate with one or two rotating engraved disks, for finding the day of the week for any date (over a wide range of years). They are often combined with portable dials as part of a compendium. More sophisticated versions have extra tables for Saint’s Days and similar data.

the approximately monthly variation of the angular separation of the Sun and the Moon, leading to the sequence of new, waxing, full and waning moons. The **age** (as seen, for example, in tables associated with moon dials) is measured in days since the last new moon. Astronomically, the phase of the Moon is defined as the angle between the Sun and the Moon measured from the Earth (the lunar angle). The mean length of the synodic (i.e., lunar) month is 29.53059 days (usually approximated to 29½ days in the lunar mechanisms of clocks).

**photosphere:**

The outer envelope of the Sun which produces the visible light by which it is seen.

**pinnules**

Sighting pinholes (usually in pairs) in an alignment device, e.g. an alidade.

**planet**

Astronomically, a celestial body in orbit around a star. The five planets of the solar system known to the ancients were Mercury, Venus, Mars, Jupiter and Saturn. In addition, they often counted the Sun and the Moon as planets; for example, in the planetary hours system. See Appendix VIII for symbols.

planetarium:

see orrery.

**planisphere**

A map of part of the celestial sphere, formed by a stereographic projection of the sphere onto a flat plane and showing (or adjustable for) the positions of the stars at a particular time and location.

**plinth**

The base part of a pedestal, normally resting on the ground. Note that some authors use ~ to refer to the whole of the pedestal. See Appendix VII for more details of architectural terms.

**plumb-line**

A freely suspended line with a weight (or plumb-bob) at its lower end, used for defining the vertical.

**plummet**

The form of plumb-line incorporated in a portable dial and used for levelling it. It usually consisting of a solid elongated cylinder suspended, by a joint with free movement in the horizontal axes, above a datum point.

**pobble**

The bead on the plumb-line of a card dial.

**polar axis**

See axis.

**polar co-ordinates**

See co-ordinates.

**polar distance**

The distance (as an angle) of the Sun from the elevated celestial pole; the complement of the declination.

**polarised light**

Light in which the electromagnetic waves have a single plane of vibration in a direction perpendicular to the direction of propagation. Polarising filters allow the transmission of light rays with only a selected plane of polarisation. Discovered by Christiaan Huygens (1635-1703). Sunlight is randomly polarised, but skylight is partially plane polarised, with the direction of polarisation at any point in the sky being perpendicular to the plane containing the point, the Sun and the observer. The proportion of the skylight which is polarised is a maximum in the principal plane and at 90º to the sun. The proportion is always less than 75%, and substantially less in slightly hazy conditions.

**Polaris (or Pole Star)**

Actually a Ursae Minoris, it is the star which appears quite close to the N celestial pole and is frequently used for finding north by navigators. It currently appears to rotate daily around a circle of radius 1º, so it requires some knowledge if it is to be used for aligning a sundial. The size of this circle varies over the centuries with the precession of the equinoxes.

**polar plane**

Any plane which is parallel to the Earth’s axis.

**polar triangle**

The spherical triangle on the celestial sphere whose vertices are at the pole, the zenith, and a celestial body, with respective angles of the hour angle, the azimuth, and the parallactic angle. The arcs joining these are the co-latitude, the north polar distance (90º -

d) and the zenith distance. The polar triangle is fundamental to the operation of most types of sundial, whose function it is to derive the hour angle, and hence the time, given any three of the other quantities.

): the locations on the Earth’s sphere with latitudes of +90º (N) and –90º (S).

**polos**

An old term for a polar-pointing style.

**post meridiem (p.m.)**

The portion of the day between noon and midnight.

**precession (of the equinoxes)**

The slow westward progression of the equinoxes on the ecliptic. It is caused by the drift of the Earth’s axis in space, as in a precessing spinning top. The position of Polaris turns around the pole of the celestial pole once in about 26,000 years. As a consequence, the vernal equinox regresses by about 50 arcseconds per year along the ecliptic. It is caused predominantly by the gravitational force of the Sun and the Moon on the Earth’s equatorial bulge. Secondary effects, due to the other planets, give a rotation of the ecliptic plane of 47 arc-seconds per century.

The first measurement of precession was made by Hipparchus in 129 BC.

**precision (of a dial)**

A combination of the resolution and accuracy of a dial, it gives a measure of how exactly (and correctly) it indicates any time.

**Prime meridian**

The meridian line defined as the origin for longitudes. Now synonymous with the Greenwich meridian, before 1884 various countries defined their own origin. The early Greeks used Rhodes or Alexandria. Ptolemy used the Fortunate Islands, assumed to be Ferro in the Canary Islands by scholars in the Renaissance. Nuremberg was common for dials made there, and in relatively modern times many maritime nations had their own locations; Paris in particular continued to be used even post-1884. Note that the 0º longitude line used by the GPS system is actually a mean value, periodically recalculated to allow for tectonic drift etc. and currently lies approximately 38 m (80 feet) east of the Greenwich line.

**Prime vertical**

The vertical circle perpendicular to the meridian. It passes through the E and W points.

**Primum Mobile**

An old term for the supposed crystal sphere carrying the stars in their orbits around the earth.

**principal plane {or vertical plane}**

The plane obtained by varying the Sun’s altitude whilst its azimuth is constant. Perpendicular to the almucantar.

**prosthaphaeretical arc**: a term introduced by Samuel Foster to describe an arc on the surface of the earth

between the location of an inclining/declining dial and the position where it would be identical to a

horizontal dial (i.e. the complementary dial). In astronomy, prosthaphaerisis is the adding of a small

amount to an observed value.

**Ptolemy’s rulers**

An interconnected set of three linear scales used to measure the angular positions of stars, used particularly by Regiomontanus and the Nuremberg group in the 1460s.