Welcome to this journey across space and time. Theuniverse (Universe)is a vast, unified system composed of space, time, matter, and energy. Within this system, our closest connection is undoubtedly the Earth-Moon system and the Sun, which powers life.

1. Earth and Moon: The Bond of Gravity

Earth's shape and measurementsreveal that our home is not a perfect sphere, but ratheran oblate spheroid with a bulging equator and slightly flattened poles. Its average radius is approximately $6371\text{ km}$, with the equatorial radius being about $21\text{ km}$ longer than the polar radius. Andthe Moon (moon)as Earth's only natural satellite, is covered with impact-formedcraters, vastmariaand magnificentray systems.

2. The Sun: A Giant of Thermonuclear Reactions

The Sun is a scorching ball of gas primarily composed of hydrogen, undergoing continuousthermonuclear reactionswithin. Its atmospheric structure, from inside out, consists of:Photosphere(visiblesunspots),chromosphere(producingprominencesandflares) and the extendedcorona. The Sun’s mass is 330,000 times that of Earth, and its gravity is the core force maintaining order in the solar system.

3. The Order of the Universe

There exists an attractive force between any two objects; he named this force universal gravitation, and gravity is one manifestation of it. It is precisely this force that binds the Earth-Moon system, and enablescomets(dirty snowballs) to trail longtailsas they approach the Sun. We must always remain vigilant towardnear-Earth asteroid impact hazardsto protect our civilization.