Fundamentals of Earth's Structure, Seasons, and Celestial Events

The Four Major Spheres of Earth

• Atmosphere: The gaseous layer that surrounds the Earth.
• Hydrosphere: The layer of water formed by the oceans, rivers, and polar ice.
• Geosphere: Corresponds to the rocky area that comprises the major materials of the Earth.
• Biosphere: The zone in which life exists on Earth. It is formed by a part of the hydrosphere and atmosphere, and all the superficial part of the geosphere.

Characteristics of the Atmosphere

Earth's atmosphere is neither extremely dense (like Venus) nor extremely tenuous (like Mars). It is a very thin layer that performs several vital functions:

• Filters ultraviolet rays.
• Prevents extreme temperatures.
• Allows the formation of clouds.

The composition of dry air is approximately:

• 78% Nitrogen
• 21% Oxygen
• 1% Carbon Dioxide and other gases

Composition of the Hydrosphere

The Hydrosphere includes all water on Earth. Approximately 97% of this water is contained in the oceans, but water is also found in rivers, lakes, and as ice (glaciers and polar caps).

The Geosphere: Earth's Rocky Structure

Beneath the atmosphere and hydrosphere, our planet has a rocky layer known as the Geosphere. It is the largest of Earth's three main structural layers:

• Crust: The outermost layer of the geosphere, abundant in materials like clay, slate, granite, and basalt.
• Mantle: The middle layer.
• Core: The innermost zone of the Earth, constituted primarily of iron and nickel.

Earth's Movement and Seasons

Why Day and Night Lengths Vary

The Earth's axis is tilted, positioning the equatorial plane at an angle of 23 degrees relative to the plane of Earth's orbit. For a few months, the Northern Hemisphere receives more hours of light, resulting in shorter nights. During the other months, the Southern Hemisphere receives more hours of daylight.

Understanding Earth's Seasons

The seasons occur because the Earth moves around the Sun. The four main seasons are:

• Spring: Begins around March 21 (the Spring Equinox). On this date, the day and night are of equal duration.
• Summer: Begins around June 21 (the Summer Solstice).
• Fall (Autumn): Begins around September 23 (the Autumn Equinox).
• Winter: Begins around December 21 (the Winter Solstice).

Why Is Summer Warmer?

The Earth's axis is tilted relative to the plane of the ecliptic (the elliptical orbit) and maintains this inclination throughout the entire orbit. This tilt causes one part of the hemisphere (e.g., the Northern Hemisphere) to be tilted toward the Sun, receiving more direct sunlight, while on the opposite side of the orbit, the Southern Hemisphere is tilted toward the Sun.

Celestial Phenomena

Understanding Lunar Phases

The appearance of the Moon changes because, due to its spherical shape, the Sun illuminates only half of it at any given moment. The other half remains in darkness and is not visible from Earth, leading to the different phases we observe.

Eclipses: Concealment of Celestial Bodies

An eclipse is the total or partial concealment of one celestial body by another, preventing its visibility.

Types of Eclipses

• Partial: Occurs if only a part of the celestial body is hidden.
• Total: Occurs if the entire celestial body is hidden.

Specific Eclipse Events

• Lunar Eclipse: Takes place when the Earth is situated between the Sun and the Moon.
• Solar Eclipse: Takes place when the Moon interposes itself between the Sun and the Earth.