Posted on by Matt Williams

What is Earth’s Axial Tilt?

Earth's axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit. Credit: Wikipedia Commons

In ancient times, the scholars, seers and magi of various cultures believed that the world took a number of forms – ranging from a ziggurat or a cube to the more popular flat disc surrounded by a sea. But thanks to the ongoing efforts of astronomers, we have come to understand that it is in fact a sphere, and one of many planets in a system that orbits the Sun.

Within the past few centuries, improvements in both scientific instruments and more comprehensive observations of the heavens have also helped astronomers to determine (with extreme accuracy) what the nature of Earth’s orbit is. In addition to knowing the precise distance from the Sun, we also know that our planet orbits the Sun with one pole constantly tilted towards it.

Earth’s Axis:

This is what is known axial tilt, where a planet’s vertical axis is tilted a certain degree towards the ecliptic of the object it orbits (in this case, the Sun). Such a tilt results in there being a difference in how much sunlight reaches a given point on the surface during the course of a year. In the case of Earth, the axis is tilted towards the ecliptic of the Sun at approximately 23.44° (or 23.439281° to be exact).

Earth's axis points north to Polaris, the northern hemisphere's North Star, and south to dim Sigma Octantis. Illustration: Bob King
Earth’s axis points north to Polaris, the northern hemisphere’s North Star, and south to dim Sigma Octantis. Credit: Bob King

Seasonal Variations:

This tilt in Earth’s axis is what is responsible for seasonal changes during the course of the year. When the North Pole is pointed towards the Sun, the northern hemisphere experiences summer and the southern hemisphere experiences winter. When the South Pole is pointed towards the Sun, six months later, the situation is reversed.

In addition to variations in temperature, seasonal changes also result in changes to the diurnal cycle. Basically, in the summer, the day last longer and the Sun climbs higher in the sky. In winter, the days become shorter and the Sun is lower in the sky. In northern temperate latitudes, the Sun rises north of true east during the summer solstice, and sets north of true west, reversing in the winter. The Sun rises south of true east in the summer for the southern temperate zone, and sets south of true west.

The situation becomes extreme above the Arctic Circle, where there is no daylight at all for part of the year, and for up to six months at the North Pole itself (known as a “polar night”). In the southern hemisphere, the situation is reversed, with the South Pole oriented opposite the direction of the North Pole and experiencing what is known as a “midnight sun” (a day that lasts 24 hours).

The four seasons can be determined by the solstices (the point of maximum axial tilt toward or away from the Sun) and the equinoxes (when the direction of tilt and the Sun are perpendicular). In the northern hemisphere, winter solstice occurs around December 21st, summer solstice around June 21st, spring equinox around March 20th, and autumnal equinox on or about September 22nd or 23rd. In the southern hemisphere, the situation is reversed, with the summer and winter solstices exchanged and the spring and autumnal equinox dates swapped.

Changes Over Time:

The angle of the Earth’s tilt is relatively stable over long periods of time. However, Earth’s axis does undergo a slight irregular motion known as nutation – a rocking, swaying, or nodding motion (like a gyroscope) – that has a period of 18.6 years. Earth’s axis is also subject to a slight wobble (like a spinning top), which is causing its orientation to change over time.

Known as precession, this process is causing the date of the seasons to slowly change over a 25,800 year cycle. Precession is not only the reason for the difference between a sidereal year and a tropical year, it is also the reason why the seasons will eventually flip. When this happens, summer will occur in the northern hemisphere during December and winter during June.

Precession of the Equinoxes. Image credit: NASA
Artist’s rendition of the Earth’s rotation and the precession of the Equinoxes. Credit: NASA

Precession, along with other orbital factors, is also the reason for what is known as “length-of-day variation”. Essentially, this is a phenomna where the dates of Earth’s perihelion and aphelion (which currently take place on Jan. 3rd and July 4th, respectively) change over time. Both of these motions are caused by the varying attraction of the Sun and the Moon on the Earth’s equatorial region.

Needless to say, Earth’s rotation and orbit around the Sun are not as simple we once though. During the Scientific Revolution, it was a huge revelation to learn that the Earth was not a fixed point in the Universe, and that the “celestial spheres” were planets like Earth. But even then, astronomers like Copernicus and Galileo still believed that the Earth’s orbit was a perfect circle, and could not imagine that its rotation was subject to imperfections.

It’s only been with time that the true nature of our planet’s inclination and movements have come to be understood. And what we know is that they lead to some serious variations over time – both in the short run (i.e. seasonal change), and in the long-run.

We’ve written many articles about the Earth and the seasons for Universe Today. Here’s Why is the Earth Tilted?, The Rotation of the Earth, What Causes Day and Night?, How Fast Does the Earth Rotate?, Why Does the Earth Spin?

If you’d like more information on the Earth’s axis, check out NASA’s Solar System Exploration Guide on Earth. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded an episode of Astronomy Cast all about Earth. Listen here, Episode 51: Earth.

Posted on by Matt Williams

Why Does the Sun Rise in the East (and Set in the West)?

A sunrise from the edge of space. Credit: Project Soar
A sunrise from the edge of space. Credit: Project Soar

You may have heard the saying at some point in your life: “The Sun will still rise in the east and set in the west tomorrow.” You get the point, it means it’s not the end of the world. But have you ever wondered why the Sun behaves this way? Why does – and always has, for that matter – the Sun rise in the east and set in the west? What mechanics are behind this?

Naturally, ancient people took the passage of the Sun through the sky as a sign that it was revolving around us. With the birth of modern astronomy, we have come to learn that its actually the other way around. The Sun only appears to be revolving around us because our planet not only orbits it, but also rotates on its axis as it is doing so. From this, we get the familiar passage of the Sun through the sky, and the basis for our measurement of time.

Earth’s Rotation:

As already noted, the Earth rotates on its axis as it circles the Sun. If viewed from above the celestial north, the Earth would appear to be rotating counter-clockwise. Because of this, to those standing on the Earth’s surface, the Sun appears to be moving around us in a westerly direction at a rate of 15° an hour (or 15′ a minute). This is true of all celestial objects observed in the sky, with an “apparent motion” that takes them from east to west.

 

 

Earth's axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit. Credit: Wikipedia Commons
Earth’s axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit. Credit: Wikipedia Commons

This is also true of the majority of the planets in the Solar System. Venus is one exception, which rotates backwards compared to its orbit around the Sun (a phenomena known as retrograde motion). Uranus is another, which not only rotates westward, but is inclined so much that it appears to be sitting on its side relative to the Sun.

Pluto also has a retrograde motion, so for those standing on its surface, the Sun would rise in the west and set in the east. In all cases, a large impact is believed to be the cause. In essence, Pluto and Venus were sent spinning in the other direction by a large impact, while another struck Uranus and knocked it over on its side!

With a rotational velocity of 1,674.4 km/h (1,040.4 mph), the Earth takes 23 hours, 56 minutes and 4.1 seconds to rotate once on its axis. This means, in essence, that a sidereal day is less than 24 hours. But combined with its orbital period (see below), a solar day – that is, the time it takes for the Sun to return to the same place in the sky – works out to 24 hours exactly.

Earth’s Orbit Around the Sun:

With an average orbital velocity of 107,200 km/h (66,600 mph), the Earth takes approximately 365.256 days – aka. a sidereal year – to complete a single orbit of the Sun. This means that every four years (in what is known as a Leap Year), the Earth calendar must include an extra day.

Viewed from the celestial north, the motion of the Earth appears to orbit the Sun in a counterclockwise direction. Combined with its axial tilt – i.e. the Earth’s axis is tilted 23.439° towards the ecliptic – this results in seasonal changes. In addition to producing variations in terms of temperature, this also results in variations in the amount of sunlight a hemisphere receives during the course of a year.

Basically, when the North Pole is pointing towards the Sun, the northern hemisphere experiences summer and the southern hemisphere experiences winter.  During the summer, the climate warms up and the sun appears earlier in the morning sky and sets at a later hour in the evening. In the winter, the climate becomes generally cooler and the days are shorter, with sunrise coming later and sunset happening sooner.

Above the Arctic Circle, an extreme case is reached where there is no daylight at all for part of the year – up to six months at the North Pole itself, which is known as a “polar night”. In the southern hemisphere, the situation is exactly reversed, with the South Pole experiencing a “midnight sun” – i.e. a day of 24 hours.

And last, but not least, seasonal changes also result in changes in the Sun’s apparent motion across the sky. During summer in the northern hemisphere, the Sun appears to move from east to west directly overhead, while moving closer to the southern horizon during winter. During summer in the southern hemisphere, the Sun appears to move overhead; while in the winter, it appears to be closer to the northern horizon.

In short, the Sun rises in the east and sets in the west because of our planet’s rotation. During the course of the year, the amount of daylight we experience is mitigated by our planet’s tilted axis. If, like Venus, Uranus and Pluto, a large enough asteroid or celestial object were to strike us just right, the situation might be changed. We too could experience what it is like to watch the Sun rise in the west, and set in the east.

We have written many interesting articles about planet Earth here at Universe Today. Here’s Why Does the Earth Spin?, The Rotation of the Earth, How Fast Does the Earth Rotate?, and Why Are There Seasons?

Here’s an article from Cornell’s Ask an Astronomer about this very question. And here’s an article from How Stuff Works that explains the whole Solar System.

Astronomy Cast also has episodes on the subject, like Episode 30: The Sun, Spots and All, and Episode 181: Rotation.