Astronomy For thousands of years, Egyptian planting was ruled by the annual flooding of the Nile river. Their entire culture revolved around this act of nature, so predicting the flooding was very important. Around 3000 BCE, people noticed that the bright star Sirius first became visible in the early morning sky every year shortly before the floods would begin. They were able to use this knowledge to prepare for the floods every year. The ancient Egyptians were among the first to study the stars - the first astronomers.
How Earth Moves Earth moves through space in two major ways: rotation and revolution. Rotation is spinning. Earth rotates on its axis. Earth’s axis is an imaginary line through the North and South poles.
How Earth Moves Earth’s rotation causes day and night. As Earth rotates from west to east the sun appears to move across the sky. The sun is not really moving as we see it. Earth’s rotation makes it appear to move. It takes Earth about 24 hours to rotate once completely on its axis.
How Earth Moves The path that Earth follows around the sun is called an orbit. Earth takes one year to travel all the way around the sun in its orbit. The exact amount of time for one complete revolution of Earth around Sol is 365.242 days. Revolution = action (what it does) Orbit = path (where it does it)
Calendars A calendar is a system of organizing time that defines the beginning, length, and divisions of a year. The Egyptians were one of the first cultures to develop a calendar based on the star Sirius. It contained 12, 30-day months (based on the lunar “moonth”) with a 5-day month at the end. Remember that it actually takes 365.242 days to orbit the sun, not an exact 365. The Romans took the Egyptian calendar when it was conquered by Alexander the Great and added a Leap Year every 4 years to compensate for the difference between the Solar year and Calendar year. This still made the Roman calendar off my 11 minutes every year. After a few centuries this added up, so Pope Gregory XIII dropped 10 days off of the year 1582. With a few more changes, he created the calendar that we still use today. This is known as a Gregorian Calendar.
The Seasons on Earth Earth is tilted 23.5°. This means that at different times of the year, the norther and souther hemispheres can be either pointed 23.5° towards the sun, or 23.5° away from the sun. In summer, Earth is tilted towards the sun. Day is longer then night. Earth’s surface receives more direct sunlight, and is therefore warmer. In winter, Earth is tilted away from the sun. Night is longer then day. Earth’s surface receives less direct sunlight, and is therefore cooler.
The Seasons on Earth Summer and winter are caused by Earth’s tilt as it revolves around the sun. The change in seasons is not caused by changes in Earth’s distance from the sun. In fact, Earth is farthest from Sol when it is summer in the Northern Hemisphere.
Earth in June In June, the north end of Earth is tilted 23.5° towards the sun. This means that the south end of Earth is tilted 23.5° away from the sun. In the north, this means more direct sunlight and longer days. This adds up to more direct heat during the summer months. Conversely, the southern hemisphere receives less direct sunlight and fewer hours of daylight (longer nights). This means less heat is striking the surface, making it winter south of the Equator.
Earth in December In December, the north end of Earth is tilted 23.5° away from the sun. This means that the south end of Earth is tilted 23.5° towards the sun. In the north, this means less direct sunlight and shorter days. This adds up to less direct heat during the winter months. Conversely, the southern hemisphere receives more direct sunlight and more hours of daylight (longer days). This means more heat is striking the surface, making it summer south of the Equator.
Solstices and Equinoxes June 21 - Longest day of the year in Northern Hemisphere = Summer Solstice June 21 - Shortest day of the year in Southern Hemisphere = Winter Solstice December 21 - Shortest day of the year in Northern Hemisphere = Winter Solstice December 21 - Longest day of the year in Southern Hemisphere = Summer Solstice
Solstices and Equinoxes March 21 - Equal amounts of day and night in both hemispheres (12 hours each). 1 of only 2 times of the year where there is NO TILT wither towards or away from the sun = Vernal Equinox in Northern Hemisphere. March 21 = Autumnal Equinox in Southern Hemisphere September 22 - Equal amounts of day and night in both hemispheres (12 hours each). 1 of only 2 times of the year where there is NO TILT wither towards or away from the sun = Autumnal Equinox in Northern Hemisphere. September 22 = Vernal Equinox in Southern Hemisphere