1. What is a day?
How long is a day? How many hours? How could we tell (without a clock) that it has been one day?

2. Are You Sure? Sidereal Day Solar Day θ θ 360° 365 𝑑𝑎𝑦𝑠 ≈1° 𝑝𝑒𝑟 𝑑𝑎𝑦
What about the fact that the earth itself moves in a circle around the sun during the course of one day? It must rotate an extra few degrees (equal to the number of degrees through which the Earth travels in a day).
360° 365 𝑑𝑎𝑦𝑠 ≈1° 𝑝𝑒𝑟 𝑑𝑎𝑦

3. How many solar days in a year? How many sidereal days in a year?
In Animation
In Animation
3 solar days
4 sidereal days
On Earth
On Earth
365 solar days
366 sidereal days
Each sidereal day is
4 minutes shorter
than a solar day!
This animation shows that 3 days happen in a “year” but the planet rotates 4 times per year. (Click to stop animation). The “shadow planet” shows what would happen if the planet were stationary (an extra day per year).
solar days in a year.
sidereal days in a year.
hours in a solar day.
24 * (365/366) hours in a sidereal day = 23 hours 56 minutes 4 seconds

4. What time is it right now?
Noon in Greenwich
Image by Strebe

5. What time is it right now?
Where are you
What time is it right now?
Greenwich, England – noon
Boston, MA – 7:15:45am
Boston, MA – ?
Rio de Janeiro, Brazil – 9:07:13am
If we adjusted time as a continuous spectrum from Greenwich depending on longitude (Boston = W; Rio = W)
-1 hr
-5 hr
Image by Strebe

6. What time is it right now?
Where are you
What time is it right now? 5 4 3 2 1
Greenwich, England – noon
Boston, MA – 7:00:00am
Boston, MA – 7:15:45am
Rio de Janeiro, Brazil – 9:00:00am
Rio de Janeiro, Brazil – 9:07:13am
So we divide into zones approximately 10 degrees in width that are CENTERED on multiples of 10 degrees of longitude (so really, from 5-15, 15-25, 25-35, etc.).
Image by Strebe

7. Time Zone Time vs. “Real Time”
… which means that when I’m in Boston and my watch says noon, it’s really 15 minutes past noon local time (because the central longitude line in my zone—where the sun is actually at the meridian at noon—is west of me).

8. Seasonal Time Summer Spring/Fall Winter
During different seasons, the Sun has a different trajectory
Image by Georgina Shaw
timeinacan.org

9. Seasonal Time
On different days at the same hour, the sun appears in different places in the sky. The shape is called the analemma
Used with permission from Kevin O’Hara

10. Seasonal Time Analemma over Greece Image by Jack Fishburn

11. Seasonal Time
Sun is not just in a different place on different days at the same time, but is also moving in a different vector direction

12. Seasonal Time
Horizontal component of sun velocity (the one that decides the time) is different depending on the day as a result of the tilt of the sun.

13. Mean Sun
Red line = “mean sun” or the average position and speed of the sun
Image by Vince Sellars
timeinacan.org

14. Spring
Summer
Autumn
Winter
Obliquity adjustments

15. Elliptical Orbit of Earth
Eccentric Time
Elliptical Orbit of Earth
How does the elliptical orbit of earth affect the length of days? When is the day longer? When is it shorter?

16. Close to Earth Spring Summer Autumn Winter Far from Earth
Eccentricity adjustments
Far from Earth

17. Equation of Time is how far ahead or behind the real sun is from the mean sun on a given day and is calculated as the sum of Eccentricity and Obliquity adjustments.

18. How can we make any accurate, trustworthy measurement?
The Problem
Ok, so we don’t know what time it is,
And we don’t know how big the Earth is,
And we don’t know what shape the Earth is,
And we don’t know how fast it’s spinning.
How can we make any accurate, trustworthy measurement?

19. OK, so don’t use the Sun Noon in Greenwich Image by Strebe

20. Universal Coordinated Time
OK, so don’t use the Sun
Universal Coordinated Time 1 2 3 4 5 6 7 8 9 10 11
24-hour days (exactly)
Seconds set by atomic clock
Noon = Sun at the Prime Meridian
Leap seconds to put the Mean Sun back on track
Make only one time zone that is not based on the movement or position of the actual sun
Image by Strebe

21. What time is it right now?
Greenwich, England – noon
Boston, MA – noon (UTC)
Boston, MA – 7:15:45am
Rio de Janeiro, Brazil – 9:07:13am
Rio de Janeiro, Brazil – noon (UTC)
Basing time on UTC means all events happen at the same numerical time.
Image by Strebe