Earth and Space: Planet Earth

Our solar system began around 4.6 billion years ago as a vast, rotating cloud of gas and dust called a solar nebula. Under the force of its own gravity, this cloud began to collapse. The vast majority of the material was pulled towards the center, becoming hotter and denser, eventually igniting to form our Sun. The remaining material flattened into a spinning disc around the young Sun, known as a protoplanetary disk. Within this disk, tiny particles of dust and rock started sticking together. Through a process called accretion, these small clumps collided and merged, growing into larger bodies called planetesimals. Over millions of years, these planetesimals continued to collide and combine, sweeping up material in their paths and eventually forming the planets, moons, asteroids, and comets we see today. The inner, hotter part of the disk formed the rocky planets (Mercury, Venus, Earth, Mars), while the outer, cooler regions allowed gas and ice to accumulate, forming the gas giants (Jupiter, Saturn) and ice giants (Uranus, Neptune).

Gravity is the fundamental force of attraction that exists between any two objects with mass. The strength of this force depends on two things: the mass of the objects and the distance between them. The more massive the objects, the stronger the gravitational pull. The closer they are, the stronger the pull. The Sun contains over 99.8% of the total mass in the solar system, so its immense gravitational force dominates, keeping everything from the largest planets to the smallest asteroids in orbit. An orbit is the path an object takes as it travels around another object in space. It's a delicate balance. A planet has a tendency to travel in a straight line (due to its inertia), but the Sun's gravity constantly pulls it inwards. This continuous inward pull bends the planet's path into a stable, curved orbit. If the planet's forward speed is too slow, it will be pulled into the Sun; if it's too fast, it will escape the Sun's gravity and fly off into space.

Tides are the daily rise and fall of sea levels, caused primarily by the gravitational pull of the Moon on Earth's oceans. The Moon's gravity pulls on the entire Earth, but it pulls hardest on the side of Earth closest to it. This pull creates a bulge of water on the side facing the Moon, resulting in a high tide. Interestingly, a second high tide bulge occurs on the opposite side of the Earth. This is because the Moon pulls the solid Earth away from the water on the far side, which is 'left behind'. The areas between these two bulges experience low tides. As the Earth rotates through these two tidal bulges each day, most coastal locations experience two high tides and two low tides approximately every 24 hours and 50 minutes. The Sun also exerts a gravitational pull, but its effect on tides is less than half that of the Moon because it is so much farther away. When the Sun, Earth, and Moon are aligned (during a new or full moon), their combined gravity creates extra-high high tides and extra-low low tides, known as 'spring tides'. When the Sun and Moon are at right angles to each other relative to Earth (during a first or third quarter moon), their gravitational pulls partially cancel each other out, leading to weaker tides with a smaller tidal range, known as 'neap tides'.

An eclipse happens when one celestial body, like a planet or moon, moves into the shadow of another. There are two main types of eclipses involving the Sun, Earth, and Moon. A Solar Eclipse occurs when the Moon passes directly between the Sun and Earth, casting a shadow onto Earth. This can only happen during the New Moon phase. The Moon's shadow has two parts: a dark inner shadow called the umbra, and a lighter outer shadow called the penumbra. If you are in the path of the umbra, you will see a total solar eclipse where the Sun is completely blocked. If you are in the penumbra, you see a partial solar eclipse. A Lunar Eclipse occurs when the Earth passes directly between the Sun and Moon, and Earth's shadow is cast upon the Moon. This can only happen during the Full Moon phase. During a total lunar eclipse, the entire Moon passes through Earth's umbra. Eclipses don't happen every month because the Moon's orbit around the Earth is tilted by about 5 degrees relative to Earth's orbit around the Sun. This means that most of the time, the Moon passes above or below Earth's shadow (for a lunar eclipse) or its shadow misses the Earth (for a solar eclipse).