Glossary term: Tropic
Description: The tropics are two lines of latitude on the Earth: the Tropic of Cancer (at 23°26′11.2″ N) and the Tropic of Capricorn (at 23°26′11.2″ S).
The Sun's position in the sky, relative to stars and other celestial objects, changes over the course of a year, moving through the constellations of the Zodiac. From the March equinox to the September equinox, the Sun is in the northern hemisphere. At approximately local midday on the June solstice, the Sun is directly overhead at the Tropic of Cancer. Between the September equinox and March equinox, the Sun is in the southern hemisphere. At approximately local midday on the December solstice, the Sun is directly overhead at the Tropic of Capricorn. During the equinoxes in March and September the Sun reaches directly overhead at the equator.
The region on the Earth between the two tropics is often called "the tropics". Here the Sun is directly overhead at local midday on two days a year.
The latitude of the two tropics above and below the equator is the angle at which the Earth's axis is tilted with respect to its orbit around the Sun.
The tropics are named after the constellations of Cancer and Capricorn which the Sun appeared to pass through during the solstices two thousand years ago. Precession of the Earth's axis means that the Sun no longer appears to be in either of these constellations at the solstices.
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Term and definition status: This term and its definition have been approved by a research astronomer and a teacher
The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".
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Related Diagrams
Latitude and Longitude
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
License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons
Related Activities
Discover Earth's climate with a balloon
astroEDU educational activity (links to astroEDU website) Description: Discover the secrets of Earth's climate zones with a hands-on experience
License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons
Age Ranges:
8-10
, 10-12
Education Level:
Middle School
, Primary
Areas of Learning:
Fun activity
, Guided-discovery learning
, Modelling
, Observation based
, Project-based learning
, Social Research
Costs:
Low Cost
Duration:
45 mins
Group Size:
Group
Skills:
Asking questions
, Constructing explanations
, Developing and using models
