Welcome!
Earth Science 5/4/2020 Mr. Basani
This session will be recorded and distributed for learning purposes. Learning purposes include: a lesson review for students who are absent, students who want to review for a test, teacher growth and improvement etc.
Lessons 4.07 Lab 4.08
Seasons,
Solstice
Equinox
We can’t really feel that the Earth is orbiting the Sun
(even though the Earth is travelling 30 km/s!), and these days not many people take notice of the Earth’s orbital position (that is, people don’t take much notice of the changing of the constellations in the night sky).
Other planets have seasons too. Investigating the
reasons for Earth’s seasons will help us understand the conditions on other planets also.
We primarily notice the passing of a year by the cycle of the seasons.
https://www.youtube.com/watch?v=48cKCKnJ7jA
What is the cause of the seasons?
Before going on to the next slide, have a think about it yourself:
what is the cause of the seasons?
Clearly the seasons have something to do with the Earth’s orbit around the Sun. Yet many people are confused about why the Earth has seasons.
Is the changing seasons caused by the change in the distance between the Sun and the Earth?
Did you say DISTANCE? WRONG ANSWER!!
“The seasons are caused by the changing distance between the Earth and the Sun, and it is warmer in summer because the Earth is closer to the Sun at summer time.”
This is a very common response - and it is true that the
Earth-Sun distance does charge. The Earth’s distance from the Sun varies by about 3% during its orbit.
So could summer occur when the Earth is closest to the Sun?
The problem with this idea is that when it’s summer in the northern hemisphere, it’s winter in the southern hemisphere, and vice versa. So if this were the correct answer, it would be the same season in both hemispheres at the same time - which is not the case.
The Earth’s rotational axis is tilted by 23.5° with respect to a line drawn perpendicular to the plane of the ecliptic (the plane of the rotation of the Earth around the Sun).
23.5°
The seasons result from this tilt of the Earth’s axis of rotation.
plane of the ecliptic
Earth’s rotation axis
This is true not only of the Earth, but all other planets with tilted rotation axes, as we shall see.
The direction of the rotational axis stays (nearly) fixed in space while the Earth orbits the Sun and the hemisphere that seems to “lean into” the Sun experiences summer, while the hemisphere that “leans away” from the Sun experiences winter.
Thus the tilt of the Earth’s rotation axis naturally explains why the seasons are opposite in the northern and southern hemispheres.
southern hemisphere
“leans into” Sun
→ summer in SH summer in NH
(leans into Sun)
northern hemisphere
“leans away” from Sun
→ winter in NH
winter in SH
(leans away from Sun)
And six months later the opposite is true:
https://www.youtube.com/watch?v=Pgq0LThW7QA
Seasons
⚫ Earth’s tilt causes the seasons.
⚫ Summer occurs when the tilt is toward the Sun for that hemisphere.
⚫ The Earth’s tilt is 23.5o
⚫ We are in the Northern Hemisphere.
N
N
N
N
Checkpoint
► Which hemisphere tilts toward the Sun during our summer?
► Random fact: Earth travels through space at about 66,700 miles per hour.
In December, when the southern hemisphere is tilted towards the Sun, the southern part of the Earth receives more sunlight and experiences long summer days.
At the same time, the northern hemisphere is tilted away from the Sun and receives less sunlight, experiencing short winter days.
equator N
S Sun
If you live near the Equator, there is not
much difference between the seasons all year
round.
When the Sun is higher in the summer sky, the sunlight is more concentrated ….
Concentrated beam of Summer sunlight
… than in winter, when the Sun is lower in the sky, and the sunlight is more diffuse.
Diffuse, “spread-out”
beam of Winter sunlight
… so for the hemisphere experiencing Summer, sunlight striking the Earth is more concentrated and this helps to raise the average temperature.
The reverse is true for the hemisphere experiencing Winter.
During spring and autumn, the two hemispheres receive approximately equal amounts of sunlight.
Let’s have a look how it works during the course of a year:
Equinox (EE-kwuh-nahks)
► one of two times each year (about March 21 and September 23) when the sun crosses the equator and day and night are approximately equal length
At what 2 points does the northern and
southern hemisphere get equal amounts
of sunlight?
Solstices and Equinoxes
⚫ Equinox: An equinox is one of two opposite points on the celestial sphere where the celestial equator and ecliptic intersect. The day and night have the same duration
⚫ Solstice: A solstice is either of the two times of the year when the sun is at its greatest distance from the equator
◦ Spring Equinox ~ March 21
◦ Summer Solstice ~ June 21 LONGEST DAY IN NORTHERN EMISPHERE
◦ Fall Equinox ~ September 22
◦ Winter Solstice ~ December 21 SHORTEST DAY IN NORTHERN EMISPHERE
⚫ The dates of the equinoxes and solstices are only approximate dates.
◦ The actual length of a year is about 365 ¼ days (365 days, 5 hours, 49 minutes), not exactly 365 days. We have to add an extra day to a year every four years to keep the seasons synchronized with the seasons (leap year). Over a longer period of time, we need to skip a leap year to compensate the extra minutes we add in every leap year to keep the calendar in sync.
Celestial Equator
Ecliptic Plane
Spring Equinox
Fall Equinox Winter Solstice
Fall Solstice
Vocab check
Let’s look at the 4.08 LAB
THERE IS NO 4.09 LAB
Screen Share
Stick (H)
Stick (H)
How is the length of the shadow in the morning and in the evening compared to noon?
Position of the sun In the afternoon Position of the sun at noon
Stick 1
Mark is at height (H) Stick 2
Mark is at height (H)
Does the length of the stick change the position of the shadow given by the mark?
What Now?
4.06-4.07 Lesson 4.08 Lab
See you on Wednesday