Documents & Resources - Search Results
Search results
Horizontal Coordinate System
image
Created for the OAE
Caption: This image shows the horizontal coordinate system of an observer on Earth. On the right we see the coordinate system in the local context of the observer. The observer appears here as a point surrounded by the celestial sphere. The ground appears as a plane, while the Earth is round, we can approximate the ground in the area around the observer as a plane. The line where this plane intersects with the celestial sphere is the horizon. The point directly above the observer is the zenith, the point directly below the observer is the nadir which is hidden by the ground.
Two coordinates define this coordinate system, altitude and azimuth, hence the reason this sometimes called an alt-az coordinate system. The altitude is zero at the horizon, maximum (90°) at the zenith and minimum (-90°) at the nadir. Azimuth is the angle around the horizon. Most commonly this is defined to be zero in the direction of north. Any point on the celestial sphere can be defined by these two coordinates. However what objects appear on these position will depend on the time and the location of the observer.
On the left-hand part of the diagram, we can see that when we put the observer and their local horizontal coordinate system in a global context, that the parts of the sky the observer can see depend on their position on the Earth and on the rotation of the Earth. The zenith points perpendicular to the Earth's surface so the position on the sky the zenith and nadir correspond to is dependent on the observer's latitude. The horizon also depends on the observer's position on the Earth.
Credit: Maria Cristina Fortuna/IAU OAE
Termes del glossari:
Altitude , Azimuth , Horizon , Zenith , Nadir
Categories:
Naked Eye Astronomy
, Observational Astronomy
License: Creative Commons Reconeixement 4.0 Internacional (CC BY 4.0) Creative Commons Reconeixement 4.0 Internacional (CC BY 4.0) icons
Arxiu
( image
1.76 MB)
PDF File
(PDF file
165.78 kB)
Also available in black and white
Arxiu
( image
483.06 kB)
PDF File
(
136.70 kB)
OAE Review: Evaluation
PDF file document
Created for the OAE
Description: The OAE Review on Evaluation published in 2025
Authors: Sophie Bartlett
Language: English
Script: Latin
License: Creative Commons Reconeixement 4.0 Internacional (CC BY 4.0) Creative Commons Reconeixement 4.0 Internacional (CC BY 4.0) icons
Arxiu
(PDF file
461.93 kB)
Latitude and Longitude
image
Created for the OAE
Caption: Two views of the Earth, one viewing the Northern Hemisphere, one viewing the Southern Hemisphere. The Earth rotates around its axis, an imaginary line that runs through the Earth from the North Pole to the South Pole. The Equator is an imaginary line that is the same distant from both the North Pole and the South Pole.
The positions of two cities, Rome in the Northern Hemisphere and Sydney in the Southern Hemisphere are marked here with red dots. Arrows indicate the two spherical coordinates latitude and longitude.
Latitude marks the position north or south of the equator. Here we can see Rome has the letter N in its latitude as it is in the Northern Hemisphere and Sydney has the letter S in its latitude as it is in the Southern Hemisphere. Latitude can vary from 90° N at the North Pole to 0° at the Equator to 90° S at the South Pole.
Longitude measures the position around the equator. While the choice of the zero point for latitude as the equator seems obvious, the choice of the zero point for longitude is more subjective. By convention the zero point in longitude is the prime meridian which passes through the Royal Greenwich Observatory in the UK. This is marked here as a solid line originating at the North Pole. Longitude is measured in degrees east or west of the prime meridian. As both Rome and Sydney lie to the east of Greenwich, they have the letter E as part of their longitude. Moving west to east, longitude varies from 180° W on the other side of the world from the prime meridian to 0° on the prime meridian before reaching 180° E again on the other side of world from the prime meridian.
This diagram shows the Earth at the December solstice. Two views are presented, one viewing the Northern Hemisphere and one viewing the Southern Hemisphere about nine hours later. The shaded region shows the night side of the Earth, with the day side being lighter. As it is the December solstice, the Sun appears overhead at the Tropic of Capricorn. This is a line of constant latitude at 23°26′09.3″ S. Six months later, at the June solstice, the Sun will appear to be overhead at the Tropic of Cancer at 23°26′09.3″ N. As the Sun appears over the Tropic of Capricorn more of the Southern Hemisphere is illuminated than the Northern Hemisphere. Indeed below the Antarctic Circle (the Polar Circle around the South Pole) the Sun does not set at this time of year leading to a Polar Day. Conversely, north of the Arctic Circle (the Polar Circle around the North Pole) the Sun does not rise at this time of year, leading to a Polar Night.
Credit: Maria Cristina Fortuna/IAU OAE
Termes del glossari:
Earth's Rotation , Equator , Latitude , Longitude , Polar Circle , Earth's Axis , Tropic , Polar Day , Polar Night , Tropic of Cancer , Tropic of Capricorn , Tropics , North Pole , South Pole
Categories:
Naked Eye Astronomy
License: Creative Commons Reconeixement 4.0 Internacional (CC BY 4.0) Creative Commons Reconeixement 4.0 Internacional (CC BY 4.0) icons
Arxiu
( image
434.22 kB)
PDF File
(PDF file
449.61 kB)
Also available in black and white
Arxiu
( image
433.51 kB)
PDF File
(
358.89 kB)
Celestial Coordinates
image
Created for the OAE
Caption: Here we see how celestial coordinates map positions on the celestial sphere. The celestial sphere is an imaginary, hollow sphere that surrounds the Earth. Celestial objects appear to be attached to the inside of the celestial sphere, and the planets, Sun, and Moon appear to move slowly across it.
The celestial sphere has a spherical coordinate system similar to the latitude and longitude we see on the Earth. The celestial equator is the projection of the Earth's equator onto the celestial sphere while the north and south celestial poles are the projections of the Earth's north and south poles onto the celestial sphere.
Declination is similar to latitude on the Earth in that it marks the angle above and below the celestial equator. Declination is zero on the celestial equator, is positive to the north of the celestial equator and negative to the south of it. It is commonly measured in degrees and sometimes in radians.
Right Ascension is similar to longitude as it marks the angle around the celestial equator. As with longitude the zero point of right ascension (shown here as a dashed line) is a matter of choice. Traditionally the zero point of right ascension was the position of the Sun at the northern hemisphere vernal equinox (spring equinox) in March. It is positive and increases to the east, unlike longitude on Earth which is defined as east or west of the prime meridian. It is most commonly measured in hours, minutes and seconds or occasionally in degrees or radians.
The ecliptic marks the Sun's path across the celestial sphere when viewed from the Earth.
The Earth rotates within the celestial sphere. This leads to objects like stars or galaxies, which appear static on the celestial sphere, rising and setting when viewed from the Earth but remaining in the same position on the celestial sphere.
The Earth's axis precesses within the celestial sphere. This slowly moves the celestial equator and the celestial poles meaning that the traditional celestial coordinate system changes slowly over time meaning that stars and galaxies that do not move will have positions that change. Astronomers have used coordinate systems defined by the position of the celestial poles and equator at a fixed point in time. Now celestial positions are most commonly measured on a fixed coordinate system defined by the position of the celestial poles and equator on the 1st of January 2000.
Credit: Maria Cristina Fortuna/IAU OAE
Categories:
Naked Eye Astronomy
, Observational Astronomy
License: Creative Commons Reconeixement 4.0 Internacional (CC BY 4.0) Creative Commons Reconeixement 4.0 Internacional (CC BY 4.0) icons
Arxiu
( image
517.83 kB)
PDF File
(PDF file
203.59 kB)
Also available in black and white
Arxiu
( image
501.79 kB)
PDF File
(
202.13 kB)
International Date Line
image
Created for the OAE
Caption: The International Date Line (shown here as a solid line) is a specific meridian which roughly follows the meridian of longitude 180°. The International Date Line passes north–south between Russia and Alaska, through the Pacific Ocean including parts of Micronesia and Polynesia, and to the east of Australia/New Zealand before reaching the South Pole on Antarctica. The International Date Line marks the boundary where calendar dates change by one. Therefore, regions to the west of the International Date line are one calendar day ahead of regions to the east.
The International Date Line does not always exactly follow the meridian of longitude 180°. This is partly to stop some territory of a country falling on the other side of the International Date Line than the rest of that country (the Aleutian Islands in the United States being a good example). Some countries also choose which side of the International Date Line they lie on. Samoa changed which side of the date line it lay on in 2011. At this point the International Date Line was moved to lie to the east of Samoa having previously passed to the west of Samoa.
On the opposite side of the world the dashed line marks the prime meridian (the meridian of longitude 0°). This is a line that passes through the Royal Greenwich Observatory in the UK. All time zones are measure relative to Universal Time which is the zero point for all timezones. The time zones marked at the top of the diagram are the timezones in the shaded regions excluding the effects of daylight savings time.
Credit: Maria Cristina Fortuna/IAU OAE
Termes del glossari:
Time Zone , Universal Time (UT) , Prime Meridian , International Date Line
Categories:
Astronomy and Society
, Planet Earth
License: Creative Commons Reconeixement 4.0 Internacional (CC BY 4.0) Creative Commons Reconeixement 4.0 Internacional (CC BY 4.0) icons
Arxiu
( image
214.47 kB)
PDF File
(PDF file
88.40 kB)
Also available in black and white
Arxiu
( image
272.39 kB)
PDF File
(
88.60 kB)
