by Anthony Barreiro » Mon Jul 16, 2012 10:04 pm
neufer wrote:JuanAustin wrote:
i found it a little odd seeing a North/East legend on the photo. Is there a convention for north, south, east and west used in space? What kind of directional conventions are used for space or envisioned for space travel?
A sky map is a mirror image of an Earth map.
When you look
up in the sky in the Northern Hemisphere
to observe the planets near zenith: North is up and
East is to you left.
Please allow me to elaborate slightly on Neufer's answer:
North / South directions in celestial images generally refer to Earth's equatorial plane and the celestial north and south poles. On the surface of the earth north and south are called latitude; in the sky north and south are called declination. The celestial equator, the sky directly overhead from Earth's equator, has a declination of 0 degrees. The north celestial pole is declination + (plus) 90 degrees. Conveniently Polaris, the North Star, is less than one degree from the north celestial pole. The south celestial pole, declination - (minus) 90 degrees, is in the constellation Octans.
East / West directions in celestial images generally refer to the point where the ecliptic, the plane of Earth's orbit around the Sun, crosses the celestial equator from south to north, i.e. the point where the Sun is in the sky at the moment of the Spring equinox. On the surface of the earth east and west are called longitude; in the sky east and west are called right ascension. (Just as the decision to make the Royal Observatory in Greenwich England the zero point for longitude was completely arbitrary, the decision for the zero point of right ascension was also completely arbitrary.) Right ascension is measured in hours. The full circle all the way around Earth's sky has 24 hours of right ascension. The point of the Spring equinox (also called the first point of Aries), in the constellation Pisces, is designated 0 hours right ascension. Summer solstice is 6 hours right ascension, Fall equinox is 12 hours right ascension, Winter solstice 18 hours right ascension.
As Neufer said, when we're looking
up into the sky (or at a celestial image or map) East and West are reversed relative to directions when we're looking
down at a map of the surface of the Earth.
To slightly complicate matters, sometimes astronomers use directions relative to Earth's ecliptic plane rather than the equatorial plane, or relative to the plane of the milky way galaxy. But this would always be mentioned. If you see simply "North" and "East" as in this image, you can safely assume that these directions are relative to Earth's equatorial plane.
[quote="neufer"][quote="JuanAustin"]
i found it a little odd seeing a North/East legend on the photo. Is there a convention for north, south, east and west used in space? What kind of directional conventions are used for space or envisioned for space travel?[/quote]
A sky map is a mirror image of an Earth map.
When you look [b][u]up in the sky[/u][/b] in the Northern Hemisphere
to observe the planets near zenith: North is up and [b][u]East[/u][/b] is to you left.[/quote]
Please allow me to elaborate slightly on Neufer's answer:
North / South directions in celestial images generally refer to Earth's equatorial plane and the celestial north and south poles. On the surface of the earth north and south are called latitude; in the sky north and south are called declination. The celestial equator, the sky directly overhead from Earth's equator, has a declination of 0 degrees. The north celestial pole is declination + (plus) 90 degrees. Conveniently Polaris, the North Star, is less than one degree from the north celestial pole. The south celestial pole, declination - (minus) 90 degrees, is in the constellation Octans.
East / West directions in celestial images generally refer to the point where the ecliptic, the plane of Earth's orbit around the Sun, crosses the celestial equator from south to north, i.e. the point where the Sun is in the sky at the moment of the Spring equinox. On the surface of the earth east and west are called longitude; in the sky east and west are called right ascension. (Just as the decision to make the Royal Observatory in Greenwich England the zero point for longitude was completely arbitrary, the decision for the zero point of right ascension was also completely arbitrary.) Right ascension is measured in hours. The full circle all the way around Earth's sky has 24 hours of right ascension. The point of the Spring equinox (also called the first point of Aries), in the constellation Pisces, is designated 0 hours right ascension. Summer solstice is 6 hours right ascension, Fall equinox is 12 hours right ascension, Winter solstice 18 hours right ascension.
As Neufer said, when we're looking [b][u]up[/b][/u] into the sky (or at a celestial image or map) East and West are reversed relative to directions when we're looking [b][u]down[/b][/u] at a map of the surface of the Earth.
To slightly complicate matters, sometimes astronomers use directions relative to Earth's ecliptic plane rather than the equatorial plane, or relative to the plane of the milky way galaxy. But this would always be mentioned. If you see simply "North" and "East" as in this image, you can safely assume that these directions are relative to Earth's equatorial plane.