Goals
After Learning completing this lesson, you should be able to answer the following questions: How is the length of a day defined? What is the difference between a sidereal day and a solar day? How are Time Zones determined? How do we define the duration of a lunar cycle? How is the length of a year defined? How does the position of the Sun relative to the Celestial Equator define the seasons?
Units of Time
Many of the units of time which we regularly use are derived from basic astronomical events.
A.M. vs. P.M.
Noon is defined as the time when the Sun crosses an observer's meridian. A.M. Ante Meridian (before the Sun crosses the meridian). P.M. Post Meridian (after the Sun crosses the meridian).
Length of Day
The length of a day is related to the rotation of the Earth. The length of time from one noon until the next is a solar day. This lasts 24 hours, on average. But the Earth doesn't sit still as it rotates. It also revolves around the Sun. Relative to background stars, the Earth takes only 23 hours, 56 minutes & 4 seconds to rotate. This is known as a sidereal day.
It takes another 3 minutes, 56 seconds to reach noon again. (Note: motion is exaggerated to emphasize the effect. Actual difference in direction ~1o.)
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Length of a Solar Day
The length of a solar day varies because the orbital speed of the Earth changes during the year. Mean Solar Day = average length of a solar day (24 hours). Equation of time = apparent solar day mean solar day. Can be as large as 17 minutes difference!
Time Zones
Due to the curvature of the Earth, local noon is different for people at different longitudes. We therefore have time zones to correct for this. One full circle equals 360o. A degree can be subdivided into arcminutes and arcseconds. One full day equals 24 hours. An hour can be subdivided into minutes and seconds. (60 minutes = 1 hour, 60 seconds = 1 minute.) We divide the circumference of the Earth into 24 time zones. Each time zone is 360o/24 = 15o wide.
360o
15o
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All regions within a time zone record noon at exactly the same time, even though the Sun crosses an observer's meridian only for those located at the center of the time zone. Therefore, the Sun may cross the meridian up to 30 minutes before or after noon at your location, depending on where within the time zone you live.
The Month
The length of a month is related to the revolution of the around Moon the Earth. Like the day, the length of a month depends on how you measure it. A sidereal month is one revolution relative to background stars. (27 days 7 hours 43 minutes and 11.5 seconds) A synodic month is one revolution relative to the Sun (one lunar cycle). (29 days 12 hours 44 minutes and 2.8 seconds)
Local time zone boundaries may vary based on distributions of populations. Some countries (like China) don't even use time zones.
(Note: motion is exaggerated to emphasize the effect.)
The Year
The length of a year is related to the revolution of the Earth around the Sun. As before, the length of a year depends on how you measure it. A sidereal year is one revolution relative to background stars. (365.25637 days) A tropical year is one revolution relative to the Sun (one seasonal cycle). (365.24219 days) The difference is due to the precession of the Earth's axis.
The Seasons
The year is separated into four seasons, based on the position of the Sun relative to the celestial equator. Equinoxes twice a year where the ecliptic crosses the celestial equator. Days when daylight lasts exactly 12 hours everywhere. Vernal Equinox the Sun crosses from the southern hemisphere to the northern hemisphere. Marks the first day of spring. Autumnal Equinox the Sun crosses from the northern hemisphere to the southern hemisphere. Marks the first day of autumn. Solstices twice a year when the Sun is farthest away from the celestial o equator (at a declination of 23.5 ). Summer Solstice the Sun is highest in the sky at noon. Marks the first day of summer. Winter Solstice the Sun is lowest in the sky at noon. Marks the first day of winter.
Calendars
If a calendar year was only 365 days long, then we would have an error of roughly 1 day every 4 years. The Julian Calendar was created to take into account the fact that a year is approximately 365.25 days (not an integer number). The solution was to introduce leap years. Every 4th year, there are 366 days in a year (Feb. 29th is added). The Gregorian Calendar (our modern calendar) was created because one leap year every 4 years was not accurate enough. Since a Tropical year is not precisely 365.25 days, having exactly 1 leap year every 4 years brings the error down to 1 day in 128 years. To fully correct for this, centuries not divisible by 400 are not leap years. This brings the error down to only 1 day in 3300 years.
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