Space Physics - Earth and the Solar System

The Earth rotates on its axis once every $24$ hours, which causes the cycle of day and night as different parts of the planet face the Sun.

The Earth orbits the Sun once every $365.25$ days. The Earth's axis is tilted at an angle of approximately $23.5^\circ$, which results in the changing seasons as the Earth moves around its orbit.

The Solar System consists of the Sun (a star), eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), dwarf planets, moons, asteroids, and comets.

The four inner planets (Mercury to Mars) are rocky and metallic (terrestrial), while the four outer planets (Jupiter to Neptune) are larger and composed mainly of gas and ice (gas giants).

Gravitational field strength ($g$) varies across different planets and depends on the mass ($M$) and radius ($r$) of the planet; for example, $g_{Earth} \approx 9.8\text{ m/s}^2$ while $g_{Moon} \approx 1.6\text{ m/s}^2$.

The Sun's energy is released through nuclear fusion in its core, where Hydrogen nuclei fuse to form Helium: $4\ ^1_1H \rightarrow \ ^4_2He + 2\ ^0_{+1}e + \text{energy}$.

A light-year is the distance light travels in a vacuum in one year, approximately $9.5 \times 10^{15}\text{ m}$. This unit is used to measure interstellar and intergalactic distances.

The orbital speed of a planet or moon is determined by the radius of its circular orbit ($r$) and its orbital period ($T$).