Introduction

Terrestrial latitude refers to the angle taken from the geometric centre of the Earth between the given surface location and the plane of the Equator. Latitude angles above the Equator (towards the North Pole) are positive and angles below the Equator (towards the South Pole) are negative. As a result, latitude angles also define the relative directions of North and South on the Earth’s surface, where the direction of increasing latitude is always northwards whilst decreasing latitude is always southwards.

Figure 1 - Lines of terrestrial latitude on the Earth's surface.

Latitude angles are typically given in decimal degrees. This can be as positive or negative values, or alternatively with an N or S suffix denoting degrees North or degrees South. They are also sometimes given in degrees, minutes and seconds. Thus, the values -23°26’13.7”, 23°26’13.7”S, 23.43713889S and -23.43713889 all reference the same latitude.

For a more precise definition, consider a ray travelling from the center of the Earth to the required location in the Earth’s surface. The latitude is the angle that the ray makes with with a flat plane passing through the the center of the Earth and in-line with the equator.

Latitudes with Solar Significance

Based on the orbital relationship between the Earth and the Sun, there are a number of significant lines of latitude that are important to know and useful to understand. These important lines all result from the Earth’s axis of rotation and the fact that it is tilted by 23°26′12.6″ (23.43731°) relative to the orbital plane of the Earth around the Sun, as shown in Figure 2.

Figure 2 - One daily rotation of the Earth on the 21st of June, showing important lines of latitude that result from the tilt of the Earth's rotational axis relative to its orbital plane around the Sun.

North and South Poles

When referenced on this site, the North and South Poles refer to the geographic locations where the Earth’s axis of rotation intersects it surface. Once thought to be absolutely fixed, we now know that there are slight periodic variations of up to 9m called the Chandler wobble. The exact cause of this variation is not well understood (ref: 1), but the most likely candidates are considered to be pressure fluctuations in the deep seabed that affect ocean currents, non-uniform atmospheric pressure distributions and the slightly non-spherical nature of the Earth.

Figure 3 - The North and South Poles at latitudes ±90°.

It should be noted that there are actually several different types of North and South Pole used in physics and astronomy, some of which may be co-located and others not. The description above is for the geographic or rotational poles, but there are also celestial poles, magnetic poles and geomagnetic poles – and their relationships to one another can be very complex.

The Earth’s rotational axis and magnetic axis are reasonably closely aligned, so the North and South Magnetic Poles are located reasonably close to the North and South Geographic Poles, at least by other planetary standards. However, as shown in Figure 4, the Magnetic Poles are slowly but continuously moving around and have varied by up to 30° in latitude in the past. There has been recent concern regarding the increasingly eratic movement of the North magnetic pole away from Canada towards Siberia.

Figure 4 - Periodically recorded locations of the Magnetic North Pole relative to the Geographic North Pole, and predicted trajectory.

The important aspect of the North and South Geographic Poles is that they represent the points of maximum and minimum latitude. The latitude of the South Pole is -90° whilst at the North Pole it is 90°.

Arctic and Antarctic Circles

The Arctic and Antarctic Circles represent the boundary lines of latitude where the Sun remains above the horizon for a full 24hr period for at least one day of the year. You can see this pretty clearly in the animation shown in Figure 2. This animation shows a single daily rotation for the solstice occuring on the 21st of June. For all points on the Earth’s surface between the Arctic Circle and the North Pole, the Sun will always be above the horizon for the entire day. For all points between the Antarctic Circle and the South Pole, the Sun will always be below the horizon for the entire day.

Figure 5 - The Arctic and Antarctic Circle lines at latitudes ±66.56269°, shown on the 21st of June.

This situation entirely reverses for the opposite solstice occuring on the 21st of December, as shown in Figure 6. In this case, for all points on the Earth’s surface between the Arctic Circle and the North Pole the Sun will always be below the horizon for the entire day. For all points between the Antarctic Circle and the South Pole, the Sun will always be above the horizon for the entire day.

Figure 6 - The exposure of the Arctic and Antarctic circles at each solstice due to the axial tilt angle.

The Arctic Circle occurs at 66°33′47.4″N (66.56269°) and the Antarctic Circle at 66°33′47.4″S (-66.56269°). This angle is simply 90° subtract the axial tilt angle.

Tropics of Cancer and Capricorn

The Tropics of Cancer and Capricorn represent the maximum and minimum latitudes at which the Sun will appear directly overhead at solar noon. This occurs on the 21st of June for the Tropics of Cancer and the 21st of December for the Tropics of Capricorn. You can also think of these tropic boundaries as the intersection line on the Earth’s surface made by an imaginary ray drawn between the centre of the Earth and the centre of the Sun when the Earth does a full daily rotation on the June and December Solstices.

Figure 7 - Tropics of Cancer and Capricorn at latitudes ±23.43731°, shown on the 21st of June.

For latitudes greater than +23.43731° and less than -23.43731°, the maximum altitude of the Sun will always be less than 90°. For latitudes between the two tropic lines, there will be always be two days in the year when the Sun at solar noon is directly overhead. Exactly what date this occurs on depends on the latitude of particular location. there will be periods of the year when the Sun at solar noon occurs in both the North and South quadraspheres

The Equator

The Equator is that latitude line that is exactly mid-way between the North and South Poles, and therefore has a latitude of 0°. The Equator is also the line on the Earth’s surface at which the Sun will be exactly overhead at solar noon on each of the March and September Equinoxes.

Figure 8 - The Equator, mid-way between the two Poles and at a latitude of 0°

Useful References

  1. Gross, R.S. (2000), The excitation of the Chandler wobble, Geophys. Res. Lett, pp 2332.

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