SEASONS NOTES

Earth’s Rotation The Earth rotates on its axis (imaginary vertical line around which Earth spins) every 23 hours & 56 minutes. One day on Earth is one rotation of the Earth. Daytime on Earth is when our side of the Earth faces the sun. Nighttime on Earth is when the side of Earth we are on faces away from the sun. The Earth rotates counterclockwise, this makes the sun rise in the east and set in the west.

Earth’s Revolution It takes the Earth 365 ¼ days (or rotations) to travel or revolve around the sun once. This is called a year. The extra ¼th day that happens each year is added to the calendar every four years as a leap day in February. This extra day makes every fourth year a leap year. This keeps our calendar on schedule.

Calendars Many years ago farmers relied on calendars to know when to plant. Plant too soon or too late and your crops might fail. It was important that calendars be accurate. Leap days were not added to calendars and so after a few years the calendars would be off from the true days by several weeks. Imagine thinking it was March and time to plant when it is really June and too late to plant. It was a real problem.

Calendars The Emperor of Rome, Julius Caesar is credited with first using the leap year in 45 BC. His astronomer, Sosigenes, is the person who actually calculated the true length of a year. This made the calendars much more accurate than they had been up to that point. Pope Gregory the eighth further refined the calendar by making only years divisible by 4 eligible to be leap years.

Why do we have seasons? The Earth’s orbit around the sun is NOT a perfect circle. It is an ellipse. Seasons are not caused by how close the Earth is to the sun. In fact, the Earth is closest to the sun around January 3 and farthest away from the sun around July 4.

Why do we have seasons? Seasons are the result of the tilt of the Earth's axis. Earth’s axis is tilted 23.5°. This tilting is why we have SEASONS like fall, winter, spring, summer. The number of daylight hours is greater for the hemisphere, or half of Earth, that is tilted toward the Sun.

Why do we have seasons? Summer is warmer than winter (in each hemisphere) because the Sun's rays hit the Earth at a more direct angle during summer than during winter

Why do we have seasons? Also the days are much longer than the nights during the summer. During the winter, the Sun's rays hit the Earth at an extreme angle, and the days are very short. These effects are due to the tilt of the Earth's axis.

Seasons…in a nut shell

Solstices Solstices occur twice a year, when the tilt of the Earth's axis is oriented directly towards or away from the Sun, causing the Sun to appear to reach its northernmost and southernmost extremes, this is the Tropic of Cancer and the Tropic of Capricorn.

Solstices

SOLSTICES Winter solstice is the shortest day of the year in the Northern Hemisphere. It occurs on December 21 and marks the beginning of winter. The Summer Solstice is the longest day of the year. It occurs on June 21 and marks the beginning of summer. June 21 marks the first day of summer, and December 21 marks the first day of winter.

Equinoxes There are only two times of the year when the Earth's axis is tilted neither toward nor away from the sun, resulting in a "nearly" equal amount of daylight and darkness at all latitudes. These events are referred to as Equinoxes.TEquinox literally means "equal night“. 12 hours of sunlight, 12 hours of darkness. Equinoxes happen when the sun crosses the celestial equator, moving from the Northern Hemisphere to the Southern Hemisphere, or vice versa. These two crossings are very important for the inhabitants of Earth, because they mark the change in the direction the sun's rays fall on Earth.

Equinoxes The vernal (spring) equinox occurs March 21 and marks the first day of spring. The autumnal (fall) equinox occurs September 21 and marks the first day of fall.

The Earth's seasons are not caused by the differences in the distance from the Sun throughout the year.

The seasons are the result of the tilt of the Earth's axis.

OBJECTIVES Differentiate between solstices and equinoxes. Understand why the Earth’s tilt contributes to its having seasons.

IN QUESTION When it is the summer solstice, June 21, in the northern hemisphere, what is happening in the southern hemisphere? The winter solstice.

VOCABULARY Solstice – when the tilt of the Earth towards or away from the sun is greatest. Equinox – when the Earth’s axis is not tilted towards or away from the sun.

VOCABULARY Tropic of Cancer – Northernmost point where the sun shines directly on the Earth. Tropic of Capricorn – Southernmost point where the sun shines directly on the Earth. Equator – Imaginary line that runs east and west around the center of the Earth.

NOTES Earth rotates on its axis every 24 hours. One year equals 365 ¼ day. Julius Caesar added the leap day to the calendar. Leap year happens every 4th year. Earth’s orbit is an ellipse.

NOTES Season’s are result of Earth’s tilt. Earth is tilted 23.5 degrees Draw this diagram.

NOTES Summer solstice is longest day of the year, June 21 Winter solstice is shortest day of the year, December 21 Vernal equinox is 1st day of spring, March 21. Autumnal equinox is first day of fall, September 21.

OUT QUESTION Why would the year 2017 not be a leap year? When is the next leap year? It is not evenly divisible by 4. The year 2020