Earth Shape, Rotation, Seasons, Moon and Eclipses

Historical Beliefs About Earth's Shape

Humanity has long believed that the Earth was flat. The sea surface appears to be horizontal, and so do the great plains of the continents.

Images taken from spacecraft show an Earth-shaped sphere. But the idea existed long before these ships were available; earlier observations seemed to confirm that fact.

The Greek philosopher Aristotle made early observations supporting a spherical Earth.

Horizon and Celestial Observations

On clear days you can see how boats disappear over the horizon. Sometimes you see the Earth's shadow projected on the Moon. It is always circular in shape, which is the type of shadow produced by a sphere. As we move north or south, we see new stars appear and others disappear from view.

Foucault Pendulum Demonstration

Léon Foucault, a nineteenth-century scientist, set out to demonstrate the rotation of the Earth. He needed a fixed reference that did not rotate with the planet. Foucault knew that a pendulum always oscillates in the same plane, but the ground beneath it rotates with the Earth, so he suspended a pendulum to show that the plane of oscillation appears to rotate relative to the surface.

Earth Rotation and Orbit

The Earth completes one rotation on its axis every day. At the same time, it moves around the Sun, completing one orbit in a year. The Earth's rotation on its axis is the cause of the succession of days and nights.

Seasons in Spain

In Spain, there are four seasons:

• Spring: begins around March 21, known as the spring equinox.
• Summer: begins around June 21, the summer solstice.
• Fall (Autumn): begins around September 23, the autumn equinox.
• Winter: starts around December 21, the winter solstice.

Axial Tilt and Day Length

If the Earth's axis of rotation were perpendicular to the plane of the ecliptic, the day would last 12 hours of daylight and 12 hours of night all over the world. Furthermore, this would happen every day of the year.

But the Earth's axis is tilted: the equatorial plane is inclined about 23 degrees with respect to the plane of Earth's orbit. For some months the northern hemisphere receives more hours of daylight and therefore has shorter nights, while in other months the southern hemisphere receives more hours of daylight.

The Moon and Its Phases

The Moon is the Earth's natural satellite; it revolves around our planet and accompanies it on its trip around the Sun. The Moon has phases such as first quarter, full moon, and new moon (including the crescent phases). The phases of the Moon occur every ~29 days, a lunar month.

Eclipses: Lunar and Solar

An eclipse is a total or partial occultation of a celestial body when another body blocks its view. There are two primary types: a lunar eclipse and a solar eclipse.

Earth's Major Spheres

On Earth there are three major layers (spheres):

• Atmosphere: the gaseous layer that surrounds the Earth. It provides air, filters ultraviolet radiation, moderates extreme temperatures, and allows the formation of clouds.
• Hydrosphere: all the water on the planet.
• Geosphere: the solid part of the Earth. It is the thickest region and has three main layers: crust, mantle, and core.

Gregorian Calendar

The Gregorian calendar is used today in most Western countries. It is also called the Christian calendar because it has its starting point in the traditionally estimated birth of Christ.