For thousands of years, we have tracked the motion of the earth relative to the sun and moon. Technically, we started measuring it the other way around, with the sun and moon treated as the objects moving around the sun.
In practice, the earth gravitationally circles around the sun, and the moon gravitationally circles around the moon, but we’re effectively using the same measurement primitives since written history.
Days/Weeks
A day is the Earth making a complete 360° rotation facing the sun, and classifies into a few time periods:
- Dawn – the specific point in time when the sun crests over the horizon.
- Morning – the period of time between when the sun rises and the sun is directly above us.
- Noon – the specific point in time when the sun is directly above us.
- Afternoon – the well-lit portion of time after noontime, typically before dinner.
- Evening – the period of time when the sun is setting or set, but everyone is still awake.
- Dawn – the specific point in time when the sun disappears over the horizon.
- Night – the time when almost everyone is asleep and the sun isn’t present.
- Dawn and dusk also have designated twilight times.
- Civil twilight (sun is <6° from horizon) – artificial lighting is typically no longer required to see clearly.
- Nautical twilight (sun is <12° from horizon) – the horizon is faintly visible, but some stars can still be used for navigation.
- Astronomical twilight (sun is <18° from horizon) – astronomers may have trouble seeing faint stars and galaxies.
A week is 7 days, though we don’t entirely know why we chose 7 days.
- While the Bible indicates it, there’s too much religious contention to create historical consensus with that alone.
- An alternate theory is that 7 days most clearly matches up with the lunar cycle.
Lunar Cycle
A lunar cycle is the amount of time for the moon to completely circle the Earth, and happens approximately every 29.53 days. From Earth, it moves through phases:
- New moon – the sun is directly behind it, so it’s invisible.
- Waxing crescent – the moon has rotated leftward a little around the Earth, so the right side reflects a sliver of light from the sun.
- First quarter – the right half becomes more visible.
- Waxing gibbous – except for the left side, it’s almost possible to see the entire moon.
- Full moon – 2-3 days where the entire moon is visible.
- Waning gibbous – the right side is now a little darkened.
- Third quarter – the right half becomes more darkened.
- Waning crescent – the moon only shows a sliver of light on the left side of its surface.
The moon has created many superstitions, likely from a few reasons:
- The moon’s rotation around the Earth wobbles a bit, so it’s rarely precisely 29.53 days.
- The moon rotates on its axis for the same length of time as its rotation, so we always see the same side.
- Since the moon has the same side facing us, we have anthropomorphized a figurative “man on the moon”.
Most ancient calendars used the moon’s rotation to create a month of 29-30 days.
Solar Cycle
Beyond the moon’s rotation, we also closely observe a seasonal shift through how directly-angled our portion of the Earth sits relative to the sun. Approximately every 365.242199 days, we go through the same seasonal cycle:
- Summer – that portion of the earth faces the sun directly.
- Fall – the earth’s surface is gradually drifting away from directly facing the sun.
- Winter – the earth is askew from directly facing the sun.
- Spring – the earth’s surface is gradually drifting toward directly facing the sun.
- Most plants and animals intuitively honor these shifts.
Intercalation
There’s no easy mathematical way to track a 29.5-day interval with a 365-day one. You’ll be short about 11 days.
To that end, every calendar has to figure out how to count that discrepancy differently:
- Hebrew calendar – 12 months, with an additional month every 2-3 years to conform the lunar cycle to the solar cycle.
- Egyptian calendar – track the calendar to the rising of the star Sirius (Sothis), with every 1,460 years constituting a Sothic cycle.
- Babylonian calendar –