Time zones and seasons have a lot of implications. You can learn a lot of big ideas about the world in a short amount of time with the connections they bring up.

Background

Why Were Time Zones Needed?

Standardizing time zones was originally motivated by the railroad system in the late 1800s. The system was now expanding to move people across great distances. It was used to help standardized listed times of arrival/departure.
This is necessary because each individual town would consider “Noon” to be when the sun reached the highest point in the sky. But, as we will see below, that is different at each location on the planet. Each stop was based on a different town’s time, making  schedules confusing, something needed to be done to improve the rail system’s efficiency.
This motivation makes sense, since railroads were the first practical means for frequent travel between distant locations.

What Was the Chosen Standard?

During a meeting between many countries met to decide what should be the starting point for time zones.
The decision was Greenwich England, which was selected as the “International Standard for 0 Degrees Longitude”. The longitudinal line marking this 0 point is called the “Prime Meridian”.

• Greenwich Meridian Time is often referenced to as GMT
• This is what GMT – # means (GMT time, a # of time zones in a direction)
  • Positive is East (right) of Greenwich, negative is West (left)
  • GMT – 6 means 6 hours behind Greenwich’s time

Prime Meridian defines the 0 point for time zones

• The “International Date Line” is located on the opposite side of the globe from the Prime Meridian (at 180°). It separates one calendar day from another and is zigzagged to avoid cutting through countries.

Time zones and seasons are different all around the world

• This 15° increment is due to Earth’s rotation speed being about 15° every hour
  • I.e. a full day, 24 hours, is a full rotation
• A meridian defines the center point of a time zone, 15 / 2= 7.5° are on either side
• Time zones in the East of Prime Meridian are “ahead in time” (relative to Prime Meridian) and thus marked with incrementing #s (positive), those West of Prime Meridian are “behind in time”

This was the original standard, however, some countries don’t follow this due to reasons such as trying to fit a country all in one time zone. For example, China has only one time zone, despite having 5 deviations of the original standard.

An interesting fact derived from this:

• China’s width is approximately 5 * 15 = 75 degrees longitude, or 15 / 360 ~= 1/5 the globe’s circumference.

Okay, that is some history behind how time zones came to be. But thinking of both time zones and seasons can help cement some of these ideas.
Here are some important points about the Earth’s rotation:

• The Earth moves and spins at the same time
  • Earth rotates once every day (24 hours)
  • Earth revolves around the sun once every year (365 days)

The below image shows a few important points:

1. Earth is tilted (23.5°), this tilt is maintained (no wobble) as it goes around the sun
   1. This is important for the seasons. Northern hemisphere is
2. The sun lights up only a part of the Earth at any time
3. This is a representation of the “December Solstice”
   1. This just means a time when Earth has a maximum tilt away from the sun

Time zones and seasons are impacted by sun exposure.

• The area of the planet that gets the most direct sunlight depends on where the Earth is at in its revolution around the sun and where you are at on Earth,
  • Direct vs indirect sunlight makes a large difference to the temperatures since, much like a flashlight, a direct shine will be warmer than if you tilt it slightly away.
  • Indirect sunlight has the amount of heat as a direct sunlight, but it is spread out across a larger area. The result is it doesn’t feel as hot.
• Since the United States is in the Northern Hemisphere, it will receive the most sunlight when the earth is tilted towards the sun.
• The seasons are due more to the tilt of the Earth than the distance from the sun
  • Earth’s revolution is roughly circular around the sun, an equal distance away.

By point 2: Due to the tilt and that the sun can only light a part of the Earth at a time…

• Some areas of the planet have a longer night (or day) than others, depending on the season. Notice how exaggerated this is near the poles.
  • That is why the poles have such extreme day/night cycles throughout the year.

By point 3: Since the “December Solstice” is when the Earth’s tilt is furthest from the sun…

• For the Northern Hemisphere, this is the “Winter Solstice”
  • E.g. North America
• For the Southern Hemisphere, this is the “Summer Solstice”
  • E.g. Australia

So, seasons came about from the Earth’s position and its direct exposure to the sun. Time zones and seasons both play a role in the modern standards for time today.

Timezones

Big Idea: You can derive times, directions, and behaviors from the background
As an example, consider the following:

• You’re in North America.
• The continent’s map is oriented as such, with the top being the North.

Orientation of continents, impacting both time zones and seasons

Each location around the globe experiences a different time zone and season.

• This is viewed from the top. You’re looking at the Norther Hemisphere.
• The middle marks the “North Pole”, which is where polar bears live
• Earth rotates counterclockwise when viewed with the North Pole on top.
  • In other words: Earth rotates “eastward” when viewed from the side.
• “Noon” is the point where the sun is “highest in the sky” (directly overhead).
• “Midnight” is the opposite point, where sun is furthest away.

NOTE: Because of the direction of spin, you’ll see the sun first coming from the East.

• This is because the Earth rotates from left to right (from the sun’s perspective)
  • So a Country more right than another Country (from the sun’s perspective) will see the sun sooner than things to the left, as the rotation occurs.
• This means the sun “rises in the East, and sets in the West”.
  • This is true for any (relatively) fixed object in the sky: Moon, Planets, Stars
• You can use this to derive your directions (i.e. North/East/South/West)

Sun exposure plays an important role in both time zones and seasons. By understanding how the Earth is exposed to sun, we can understand both time zones and the seasons together.
With this in mind…

The Punchline

• If you are in the central United States, you can derive the time in California.
  • Roughest Approximation:
    • You’re looking at your location from a “birds-eye” view, from the perspective of the sun when it is highest in the sky.
    • California is on the West Coast
      • To the “left” with above perspective
    • The Earth rotates counterclockwise
      • “Eastward”, or from left to right with above perspective
    • Since California is to your left, and the rotation is from left to right, you will see the sun ahead of California in the same day.
    • Therefore: California’s time is in the past
  • More Accurate:
    • Consider the below image of United States with longitude lines
      • Recall: Each time zone is marked in 15° increments around the globe
      • NOTE: The below map has longitude lines at 10° increments

• • • You can see from Kansas (Central United States), to California is a little more than 2 Longitudinal lines over, or approximately 25-30°
    • So, California is approximately 2 time zones over from Kansas
    • Each time zone is an hour change, we are going from right to left and thus going “back” in time (to a point where the sun

Great, so both time zones and seasons relate to sun exposure.

Interesting Connections

Temperature Difference: North vs South Poles

Both the North and South poles are cold because they don’t get direct sunlight.
On the same token, locations around the equator (the “tropics”) are very warm due to receiving direct sunlight.

Polar Bears and Penguins

Polar bears live in the North Pole and Penguins in the South Pole, despite what fiction movies might portray (probably because they look cute together).
The North Pole is also slightly warmer than the South Pole.
I remember this by thinking “heat rises“.

Afterword

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