An Introduction to Planet Earth

Let's revise some basic concepts

Hello and welcome to the very first post of the Geoscience section! We will talk more about the scientific aspects of our planet: to enjoy a good hike, you should know something about what you're stepping on or what's around you. So, I'd like to start off with a little introduction to the dynamics of our atmosphere; there's so much going on yet we take too many things for granted, so I want to make a couple of things clear before diving into the more interesting facts, so you have a solid base for future posts, so nice of me. Let's start!

• Earth's characteristics

The Earth is not round. Oh, don't panic! That's just because it's an "oblate spheroid" - meaning our planet is slightly flattened at the poles and bulges at the equator! And, speaking of poles, they're the points where Earth's axis passes through (North and South), and they're related to our magnetic field too - like a giant magnet, but that's a story for another time.
On the other hand, the equator is the imaginary line circling the middle of our planet, dividing it into the northern and southern hemispheres. Positioned at 0° latitude, it receives the most direct sunlight year-round and experiences nearly equal day and night.

Earth's rotation is responsible for our day/night cycles. Our planet spins on its axis like a top, completing one full rotation every 24 hours. The side facing the sun experiences daylight, while the side facing away is in darkness. This constant rotation creates the familiar rhythm of sunrise and sunset. The axis, though, is tilted at ~23.5°, so Earth isn't rotating "vertically" but at an angle—and that tilt is what gives us our seasons! 

This tilt also means different parts of Earth receive varying amounts of sunlight throughout the year. When the Northern Hemisphere is tilted toward the sun, it experiences summer, while the Southern Hemisphere simultaneously experiences winter, and vice versa, creating our solstices and equinoxes: solstices occur around December 21 and June 21, marking when one hemisphere is maximally tilted either toward or away from the sun. The December solstice brings the shortest day to the Northern Hemisphere and the longest to the Southern, while the June solstice does the opposite. Equinoxes happen around March 20 and September 22, when both hemispheres receive equal daylight hours.

The tilt even creates two important latitude circles: the Tropic of Cancer (23.5°N) and the Tropic of Capricorn (23.5°S). These mark the northernmost and southernmost points where the sun appears directly overhead during the June and December solstices, respectively. The region between these tropics receives the most direct sunlight and typically experiences wet and dry seasons rather than four distinct seasons.

Earth also revolves around the sun in an elliptical orbit, completing one full circuit every 365.25 days. During this orbit, our planet reaches perihelion (closest approach to the sun) in early January and aphelion (farthest point) in early July. Interestingly, the Northern Hemisphere experiences winter during perihelion, showing that axial tilt, not distance from the sun, primarily determines our seasons.

• Orientating ourselves

For navigation and orientation purposes, we use latitude and longitude. Latitude lines, also called parallels, run east-west around the globe, parallel to the equator. They measure distance north or south from the equator (0°) to the poles (90°). Longitude lines, or meridians, run north-south from pole to pole, measuring east or west from the Prime Meridian in Greenwich, London (0°) to 180° on the opposite side of the planet. Together, these lines form a grid system that allows us to pinpoint any location on Earth.

• Time zones

Meridians and time zones share a fascinating relationship that's directly tied to Earth's rotation. Since our planet makes a full 360-degree rotation in 24 hours, it turns at a rate of 15 degrees per hour. Each standard time zone is ideally based on a 15-degree section of longitude, centred on a meridian that's a multiple of 15° from the Prime Meridian in Greenwich. For example, the time zone for Eastern Standard Time is centred around the 75°W meridian, while Central European Time is based on the 15°E meridian. When the sun is directly over a particular meridian, it's solar noon for all locations along that line of longitude. As you travel eastward across meridians, local time advances by four minutes per degree of longitude. This is why, theoretically, there should be 24 time zones of 15 degrees each. However, in practice, time zone boundaries rarely follow meridians perfectly. They zigzag to accommodate national borders, regional preferences, and practical considerations. Some countries, like China, even use a single time zone despite spanning multiple 15-degree sections. Still, the underlying mathematical relationship between meridians and time remains the foundation of our global timekeeping system. And which place sees the new day first? That honour goes to the islands of Kiribati and Samoa in the Pacific!

So next time you're huffing and puffing up that mountain trail, take a moment to appreciate that you're not just conquering a hill – you're riding on a giant, squashed cosmic carousel that's spinning through space at breakneck speed while chasing the sun ;)

-A.