The angle between the directions in which an object is seen from two different positions. The parallax of an object seen with the left and right eye helps create depth perception. The stellar parallax (stellar=of a star) is the angle between the directions a star appears to us, when viewed from opposite sides of the Earth's orbit, half a year apart. Even though that distance is 300 million kilometers, the stars are so much more distant that even for the closest star the parallax is only 3/4 of a second of arc. See parsec.


(From PARallax + SECond). A unit of distance between stars. A star would be one parsec from Earth if its (stellar) parallax (see above) were 1 second of arc. One parsec is about 3 1/4 light years.


in general, a charged component of an atom, that is, an ion or electron.


the point of a satellite's orbit closest to Earth (see perihelion, apogee).


The point in a planet's orbit when it is closest to the Sun (Helios is Greek for Sun). See aphelion, perigee

Persian Calendar

A calendar used in Iran and some of its neighbors. It is a solar calendar, counting its years from the same beginning as the Moslem calendar. See Naw Ruz.


colloquially, a "particle of light." Although light spreads as an electromagnetic wave, it can be created or absorbed only in discrete amounts of energy, known as photons. The energy of a photon is greater the shorter the wavelength--smallest for radio waves, increasingly larger for microwaves, infra-red radiation, visible light and ultra-violet light. It is largest for x-rays and gamma rays.


The layer of the Sun from which all visible light reaches us. The Sun is too hot to have a solid surface and the photosphere consists of a plasma at about 5500 degrees centigrade.

Plane of the ecliptic

(also called "the ecliptic" for short) The orbital plane of the Earth around the Sun. The line of the ecliptic on the celestial sphere is formed by the intersection of the plane of the ecliptic with that sphere. The reason the major planets and Moon appear in the sky close to the ecliptic is that the solar system is flat, and its orbital planes are very close to each other. We observe their motion (very nearly) edge-on.


Celestial bodies such as the Earth which orbit the Sun (and by extension, similar orbiters around distant stars). Counting from the Sun outwards, planets visible to the eye are Mercury, Venus, (Earth), Mars, Jupiter and Saturn. The telescope also sees the more distant Uranus, Neptune and Pluto, as well as smaller asteroids (most of them inside the Jupiter orbit) and Kuiper objects (in the outer solar system). See also retrograde motion

Planetary swing-by maneuver

The encounter between a moving spacecraft and a moving planet or moon, affecting the spacecraft's motion like an elastic collision (in which no energy is lost to heat). Depending on the details of the encounter, the spacecraft can gain or lose appreciable amounts of energy, and appreciable changes in the direction of its motion can result.

Swing-by maneuvers with the Moon have been used to reach the L1 Lagrangian point; fly-by maneuvers with the planets have played an essential role in space missions exploring the solar system. Lunar fly-by is similar and is also used.


a gas containing free ions and electrons, and therefore capable of conducting electric currents. A "partially ionized plasma" such as the Earth's ionosphere is one that also contains neutral atoms.

Polar Coordinates

An alternative system of marking a point on a plane by its radial distance (r) from an "origin" and a polar angle (f). Polar coordinates in 3-dimensional space use (r) and two polar angles (q,f) giving the direction from the origin to the point.

When 3-dimensional polar coordinates overlap a cartesian (x,y,z) system, q is the angle between the line to the origin and the z-axis, while f is the angle (counter-clockwise when viewed from +z) between the projection of that line onto the (x,y) plane and the x-axis. Concerning (q,f), see also latitude and longitude, declination and right ascension, azimuth and elevation.

Polaris (Pole Star, North Star)

A fairly bright star, the last star in the tail (or handle) of the constellation of the Little Dipper (Ursa Minor). Currently located within a fraction of a degree from the celestial north pole, the point around which the celestial sphere appears to rotate. In the northern hemisphere, the direction towards Polaris is very nearly due north.

Potential energy

Energy stored in the set-up of a mechanical system--e.g. by a weight able to descend (in the presence of gravity), or by a compressed spring.


The rate at which energy is supplied. See watt.


A modern term, derived from the precession of the equinoxes and meaning a motion around a cone of the rotation axis of a spinning body.

Precession of the Equinoxes

A slow motion of the axis of the Earth around a cone, one cycle in about 26000 years. As a result, the celestial pole moves around a circle in the sky, and in ancient times, for instance, was quite far from Polaris. Discovered by Hipparchus around 130 BC as a slow shift of the vernal equinox around the ecliptic (i.e. around the zodiac).


A cloud of cooler plasma extending high above the Sun"s visible surface, rising above the photosphere into the corona.

Propeller pitch

the angle at which the propeller blade (or part of it) "bites" into the air, its angle of attack.


an ion of hydrogen and one of the fundamental building blocks from which atomic nuclei are made.

Ptolemy's System

The explanation given by ancient Greek astronomers to the motion of planets around the sky, described in a book by the Greek Ptolemy, around 150 AD. It regarded Earth as the center of the universe and assumed the motion of planets was a superposition of circular motions (see epicycles).


See neutron star

Pythagoras, theorem of

A proved assertion in geometry, that in a right-angled triangle which has sides of length (a, b, c), if c is the long side facing the right angle, then a2 + b2 = c2