What is an Equinox? - Definition & Types

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• 0:00 What Is an Equinox?
• 1:03 Types of Equinoxes
• 1:31 Path of the Sun on the Equinox
• 2:54 Lesson Summary
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Lesson Transcript
Instructor
David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

Expert Contributor
Will Welch

Will has a doctorate in chemistry from the University of Wyoming and has experience in a broad selection of chemical disciplines and college-level teaching.

After completing this lesson, you will be able to explain what the equinox is, describe the types of equinox, and what is special about the path of the Sun through the sky on the equinox. A short quiz will follow.

What Is an Equinox?

It's common knowledge the days are longer than nights in summer, and that in winter, the days are shorter. However, this is not quite true during some parts of the year: during equinoxes. An equinox is a point in the year when daytime and nighttime are exactly the same length, 12 hours each. Equinoxes occur twice a year, usually on March 20 and Sept. 22, when the Earth is tilted neither toward nor away from the sun. The word is derived from Latin: 'equi,' which comes from aequus, meaning equal, and nox-, meaning night.

In actuality, day and night are not exactly the same length on the equinox, but they are very close. There are various reasons for this minor difference in duration. The way we define day and night uses the edge of the sun instead of the center as a point of measurement. Also, refraction allows people to see sunlight sooner than the sun actually rises above the horizon.

Types of Equinoxes

There are only two types of equinoxes. The March and September equinoxes each have their own names. In the Northern Hemisphere, the March equinox is called the vernal equinox, while in September, it is called the autumnal equinox. In the Southern Hemisphere, the names are the opposite because the seasons are switched. For example, autumn and the autumnal equinox occur in the Southern Hemisphere in March, when it is spring in the Northern Hemisphere.

Path of the Sun on the Equinox

When you get up each morning, take a look at where the sun is in the sky. It's probably somewhere in the East. Just before it goes dark outside, the sun is located westward. As many learned in primary school, the sun rises in the East and sets in the West. But it isn't quite that simple. If you're in the Northern Hemisphere, the sun will follow an arc that's a little off to the south during the day. Whether it's nearly overhead or really far south depends on two things: your latitude and the time of year.

Latitude is the angular distance, measured in degrees, of a place north or south of the Earth's Equator. A latitude of zero degrees is on the Equator of the Earth; 90 degrees south is the South Pole and 90 degrees north is the North Pole.

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Recall that the sun is directly overhead during equinoxes only at the equator.

• In North America, at noon, what direction do shadows point during the equinoxes?
• How does the specific direction change throughout the day?
• What about at the North Pole?
• Do the shadows get longer or shorter as the planet moves away from each equinox?

• North America is at a high enough latitude that the sun is always at least slightly to the southern part of the sky, so shadows will always point north to some extent, although they may point northeast of northwest depending on the time of day. At noon during the equinox, they will point due north.
• In the morning the sun is in the southeast, so the shadows will point toward the northwest. After noon, the shadows begin to drift toward the northeast.
• At the true North Pole, all directions pointing away from it are south, so shadows will always point south at the North Pole.
• The Earth is tilted so that the northern hemisphere points toward the sun during summer and is highest in the sky at the summer solstice (June 21). So shadows get shorter as the planet moves away from the spring (vernal) equinox. Similarly, the sun is lowest in the sky during the winter solstice, and shadows are longest in the norther hemisphere in the winter. So as the planet moves away from the autumnal equinox, the shadows are getting longer.

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