What is a day?
Are You Sure?
Sidereal Day
Solar Day
θ
θ
360°
≈ 1° 𝑝𝑒𝑟 𝑑𝑎𝑦
365 𝑑𝑎𝑦𝑠
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!
What time is it right now?
Image by Strebe
https://en.wikipedia.org/wiki/List_of_map_projections#/media/File:Equirectangular_projection_SW.jpg
Where are you
What time is it right now?
Greenwich, England – noon
Boston, MA – 7:15:45am
?
Rio de Janeiro, Brazil – 9:07:13am
-5 hr
-1 hr
Image by Strebe
https://en.wikipedia.org/wiki/List_of_map_projections#/media/File:Equirectangular_projection_SW.jpg
Where are you
What time is it right now?
Greenwich, England – noon
7:15:45am
Boston, MA – 7:00:00am
5 4 3 2 1 0
9:07:13am
Rio de Janeiro, Brazil – 9:00:00am
Image by Strebe
https://en.wikipedia.org/wiki/List_of_map_projections#/media/File:Equirectangular_projection_SW.jpg
Time Zone Time vs. “Real Time”
Seasonal Time
Summer
Spring/Fall
Winter
Image by Georgina Shaw
timeinacan.org
Seasonal Time
Used with permission from Kevin O’Hara
https://kmo6un.wordpress.com/2013/09/12/assignment-1-sun-path-diagram-2/
Seasonal Time
Image by Jack Fishburn
https://en.wikipedia.org/wiki/Analemma#/media/File:Analemma_fishburn.tif
Seasonal Time
Seasonal Time
Mean Sun
Image by Vince Sellars
timeinacan.org
Time Adjustment due to Obliquity
20
Minutes from Mean Sun
15
10
5
Spring
0
0
50
Autumn
Summer
100
150
200
-5
-10
-15
-20
Days
250
Winter
300
350
Eccentric Time
Elliptical Orbit of Earth
Time Adjustment due to Eccentricity
20
Minutes from Mean Sun
15
10
Close to Earth
5
Spring
0
0
50
Autumn
Summer
100
150
200
-5
Far from Earth
-10
-15
-20
Days
250
Winter
300
350
Equation of Time
20
Minutes from Mean Sun
15
10
5
0
0
50
100
150
200
250
300
350
-5
-10
Obliquity
Eccentricity
-15
Sum
-20
Days
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?
OK, so don’t use the Sun
Image by Strebe
https://en.wikipedia.org/wiki/List_of_map_projections#/media/File:Equirectangular_projection_SW.jpg
OK, so don’t use the Sun
• 24-hour days (exactly)
• Seconds set by atomic clock
11
10
9
8
7
6
5
3
2
1
2
3
4
1
0
Universal
Coordinated
Time
4
5
6
7
8
9
10
• Noon = Sun at the Prime Meridian
• Leap seconds to put the Mean Sun back on track
Image by Strebe
https://en.wikipedia.org/wiki/List_of_map_projections#/media/File:Equirectangular_projection_SW.jpg
11
What time is it right now?
Greenwich, England – noon
Boston, MA – noon
(UTC)
7:15:45am
Rio de Janeiro, Brazil – noon
(UTC)
9:07:13am
Image by Strebe
https://en.wikipedia.org/wiki/List_of_map_projections#/media/File:Equirectangular_projection_SW.jpg