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Seasons and the Sun: Crash Course Kids 11.1

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A solar eclipse is when the moon moves in between the Earth and Sun, casting a shadow on to the Earth?

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A lunar eclipse occurs when the Earth passes between the Sun and Moon, casting a shadow on the moon.

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- [Narrator] When you look at the moon in the night sky, it might seem reasonably close. But it's actually incredibly far away. Right over here is a scaled picture of the distance between the earth and the moon. Earth has a diameter of approximately 8,000 miles. While the moon has a diameter of approximately 2,200 miles. So, a little bit more than one fourth the diameter of earth. Now the distance between the two is 239,000 miles. Which you can imagine is incredibly far. Now one thing that is very interesting, this is why the moon looks like it's the same size as the sun in the sky, even though it is one four hundredth of the diameter, is the ratio between the distance to the moon and the diameter of the moon is roughly the same as the ratio of the distance of the sun to the diameter of the sun. So, for the moon, that ratio, the distance to the moon, 239,000 miles over the radius of the moon, 2,200. It's approximately the same as the ratio between the distance from earth to the sun which would be 93 million miles over 865,000 miles in diameter. So this number is roughly 400 times larger than that number. And this number is roughly 400 times larger than that number. But what's neat is, and this depends on where earth is in its orbit, but this is approximately equal to 108. This gives me goosebumps whenever I think about it because it didn't have to be this way. That the moon and the sun, even though they have these vastly different diameters, that they look roughly the same size in the sky because the ratio of the distances is comparable to the ratios of their diameters. And this is why the number 108 is actually an auspicious number in some cultures, especially in Hinduism. Now, when you're discussing the moon and especially the moon as viewed from earth, one of the obvious questions is why does the moon go through phases? Why do we see different portions of the moon lit up from day to day? This right over here is a diagram from NASA's website. And what you see here, clearly this is not at scale. This picture over here, this inner picture, the size of the moon and the earth is roughly at scale, but clearly the distances between them are not. You don't see that 239,000 miles between them. In this inner circle, what you see is that the moon and the earth is always lit up from the right. So, it's assuming that the sun is 93 million miles in that direction. And it is lighting up both the earth and the moon from the right. Now, as the moon rotates around the earth in its 28 day cycle, and if you're wondering, gee a 28 day cycle seems awfully close to a month, that is not a coincidence. The notion of a month comes from the cycles of the moon. In fact, even the word month and the word moon have the exact same root in old English. And in proto Germanic. They are essentially the same word. But what you see is the moon goes in this 28 day cycle, when the moon is roughly between the earth and the sun, the lit up half of the moon is away from what we can see here on earth. And so, we see the non lit up side which would be a new moon. Now, as the moon goes, this is viewing from above the earth, this would be the North Pole right over here, as it goes counter clockwise as viewed from above the earth, we start to be able to see a little bit of that half of the moon that gets lit up. So, when the moon is in this position, we see, us from earth, from this vantage point, we're able to see a little bit of the lit up side. When the moon is over here, we're able to see half the lit up side and half the non lit up side. And that's called a first quarter moon. And that keeps on going. When we're halfway through our cycle here, at a full moon, the earth is between the moon and the sun and so from our vantage point, we are able to see the entire lit up side. And that's why it is a full moon. And then we keep going all the way until we get back to a new moon. Now, one question that might be bothering you, it bothered me for many years, is as soon as I understood this, the cycle of the moon, how the moon has this 28 day cycle as earth rotates around the sun, I always wondered well the new moon, it looks like the moon is between the earth and the sun, why doesn't it block out the sun every time we have a new moon? Why don't we have a solar eclipse every 28 days? Similarly, for the full moons, when we're in this position, it looks like the earth is between the moon and the sun. Why doesn't earth's shadow block out the sun so that we have a lunar eclipse instead of a full moon and we would have one of those every 28 days? Why don't we see that? Think about it yourself. What's a plausible explanation? In the next video, we'll actually talk about why eclipses, both solar eclipses when the moon blocks out the sun, and lunar eclipses when the earth's shadow falls on the moon, why they're not happening every 28 days.

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Intro to Moon phases (video) | Khan Academy

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Narrator] When you look at the moon in the night sky, it might seem reasonably close. But it's actually incredibly far away. Right over here is a scaled picture of the distance between the earth and the moon. Earth has a diameter of approximately 8,000 miles.

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When do spring, summer, fall, and winter start and end? It depends on which definition you use and if you are north or south of the equator.

©bigstockphoto.com/iluzia

The Four Seasons

The Earth's axis is slightly tilted in relation to its orbit around the Sun. This is why we have seasons.
How exactly do seasons work?
In most cultures, including all western countries, the year is commonly divided into four seasons:

Since the year has 12 months, each season lasts about three months. However, the dates when the seasons begin and end vary depending on whom you ask. Two methods are most commonly used to define the dates of the seasons: the astronomical definition and the meteorological definition.
Astronomical terms & definitions Earth: The living planet
Astronomical Seasons

The astronomical definition uses the dates of equinoxes and solstices to mark the beginning and end of the seasons:
What are equinoxes and solstices?
The beginning of each season marks the end of the last.

Because the timings of the equinoxes and solstices change each year, the length of astronomical seasons within a year and between years also vary.
Equinoxes and solstices dates worldwide
Meteorological Seasons

According to the meteorological definition, the seasons begin on the first day of the months that include the equinoxes and solstices. In the Northern Hemisphere, for example,

spring runs from March 1 to May 31;

summer runs from June 1 to August 31;

fall (autumn) runs from September 1 to November 30; and

winter runs from December 1 to February 28 (February 29 in a leap year).

©timeanddate.com

Opposite Sides - Opposite Seasons

Seasons in the Southern Hemisphere are opposite to those in the Northern Hemisphere. For example, under the definition of astronomical seasons, the June solstice marks the start of summer in the Northern Hemisphere, but it is the start of winter in the Southern Hemisphere. The same rule applies for the other seasons.

spring starts September 1 and ends November 30;

summer starts December 1 and ends February 28 (February 29 in a Leap Year);

fall (autumn) starts March 1 and ends May 31; and

winter starts June 1 and ends August 31;

The Sun: Our home star
The meteorological seasons in the Southern Hemisphere are also opposite to those in the Northern Hemisphere:

Different Countries, Different Seasons

The question which definition to use divides countries and regions around the world. For example, Australia and New Zealand use the meteorological definition, so spring begins on September 1 each year. In many other countries, both definitions are used, depending on the context.

In Ireland, St Brigid's Day on February 1 is often thought to mark the beginning of spring in the ancient Celtic calendar system. Some cultures, especially those in South Asia have calendars that divide the year into six seasons, instead of the four that most of us are familiar with.

In Finland and Sweden, the dates of the seasons are not based on the calendar at all, but on temperatures. This means that the seasons within each county start and end on different dates, depending on the regions and their climate.

In Iceland, the first day of summer, a national holiday, falls on the first Thursday after April 18.

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Seasons are periods in a year marked by specific weather conditions, temperatures and length of day. Most modern day calendars divide the year in 4 seasons: spring, summer, fall (autumn) and winter.

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Earth's Rotation & Revolution: Crash Course Kids 8.1

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Ever since the 16th century when Nicolaus Copernicus demonstrated that the Earth revolved around in the Sun, scientists have worked tirelessly to understand the relationship in mathematical terms. If this bright celestial ...

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Most people grow up thinking that the tides are caused by the moon, and indeed that gravitational ‘pull’ of the
moon is a major factor, as is the gravitational effect of the sun but there is another major factor, which is
less often mentioned, and that is the force created by the rotation of the earth itself.