Cycles in the Sky: Crash Course Astronomy #3

Summary

This script explores the celestial cycles and Earth's movements, explaining how stars appear to rise and set due to Earth's rotation, and how their positions shift over weeks and months as Earth orbits the Sun. It delves into the concept of the ecliptic and the zodiac, the movement of planets, and the tilt of Earth's axis that causes seasons. The script also touches on the precession of Earth's axis and the historical significance of the stars to ancient civilizations, emphasizing the importance of observing the night sky and the joy it brings.

Takeaways

• 🌌 Under a dark sky, thousands of stars are visible and they rise and set as the Earth rotates.
• 🕒 Observing the sky over a few weeks reveals subtle changes in star positions due to Earth's orbit around the Sun.
• 🔄 The stars rise about four minutes earlier each night, making constellations visible at different times over the year.
• 🌞 The path the Sun takes through the sky reflects Earth's orbit around the Sun, forming a line called the ecliptic.
• ♈ The ecliptic passes through the zodiac constellations, which the Sun appears to move through annually.
• 🌍 Earth's 23.5-degree axial tilt causes seasonal changes in the length and intensity of daylight.
• ❄️ Seasons result from the angle of the Sun's rays, not Earth's distance from the Sun.
• 🔄 Earth's axis precesses over 26,000 years, changing the North Star and shifting the zodiac constellations over time.
• 🔭 Naked eye astronomy has been a key method for humans to understand the sky and time cycles.
• 🌠 Modern light pollution and reliance on technology have diminished our connection to the night sky and its natural cycles.

Q & A

What causes the stars to appear as if they rise and set?

The stars appear to rise and set due to the Earth's rotation on its axis once a day. As the Earth rotates, different parts of the sky come into view, creating the illusion of stars moving across the sky.

How does the Earth's position in its orbit around the Sun affect the night sky over time?

As the Earth orbits the Sun, its position changes slightly each day, altering our viewpoint on the sky. This causes the stars to appear to move their positions relative to the Sun, leading to noticeable changes in the sky's appearance over weeks, months, and a year.

What is the significance of Polaris in relation to the Earth's axis?

Polaris, also known as the North Star, is significant because it is located almost directly above the Earth's North Pole. It appears to hang stationary in the sky, while other stars rise and set around it, making it a useful navigational aid.

Why do stars appear to rise and set at different times throughout the year?

The Earth's orbit around the Sun causes stars to appear to rise and set at different times throughout the year. As the Earth moves along its orbit, the angle at which we view the stars changes, leading to a shift in their positions relative to the horizon.

What is the ecliptic and how is it related to the Sun's apparent path in the sky?

The ecliptic is an imaginary line in the sky that represents the Earth's orbit around the Sun. The Sun appears to move along this line, passing through the same constellations each year, which are collectively known as the zodiac.

How do the planets' movements in the sky differ from the stars?

While stars appear to move due to the Earth's rotation and orbit, the planets move relative to the stars themselves. Inner planets like Mercury and Venus move rapidly and can be seen to change position after a single night, whereas outer planets move more slowly, taking longer to change their positions.

What is the reason behind the changing seasons on Earth?

The changing seasons are caused by the tilt of the Earth's axis, which is 23.5 degrees relative to its orbital plane. This tilt causes the Sun's path across the sky to vary throughout the year, leading to longer days and shorter days, and thus warmer and colder seasons.

Why is the Earth's orbit not the primary cause of the seasons, despite being elliptical?

Although the Earth's orbit is elliptical, causing it to be closer to the Sun in January and farther in July, the primary cause of the seasons is the tilt of the Earth's axis. The angle of the Sun's rays, not the distance from the Sun, determines the temperature differences between seasons.

What is the phenomenon known as precession, and how does it affect the sky's appearance over long periods?

Precession is the slow, circular motion of the Earth's axis, similar to a wobbling spinning top. This motion takes approximately 26,000 years to complete and affects the position of the North Star, the dates the Sun is in a particular zodiac constellation, and other aspects of the sky's appearance.

How has the understanding of the sky evolved from ancient times to the present?

Ancient humans used the sky as a clock and calendar, observing the positions of stars and constellations to track time and seasons. Over time, the application of math and physics to these observations has allowed us to understand more about the Earth's movements, the nature of stars, and the structure of the solar system.

What is the importance of naked eye astronomy, and how has it shaped human understanding of the universe?

Naked eye astronomy, or observing the sky without the aid of telescopes, was the primary method of understanding the universe for thousands of years. It allowed early observers to track time and seasons and laid the foundation for our current knowledge of astronomy, even though some of their interpretations, like astrology, have been proven to be incorrect.

Outlines

00:00

🌌 Celestial Observations and Earth's Orbit

This paragraph discusses the observable phenomena in the night sky and how they are influenced by Earth's rotation and orbit around the Sun. It explains that while stars appear to rise and set daily due to Earth's rotation, subtle changes in their positions over weeks and months are due to Earth's orbit. The paragraph also introduces the concept of the ecliptic, the apparent path the Sun takes across the sky, which is a reflection of Earth's orbital path. It further explains how the stars' positions change relative to the Sun throughout the year, affecting the visibility of constellations and the movement of planets, which are also in orbit around the Sun.

05:00

🌞 Seasons, Earth's Tilt, and Astronomical Changes

The second paragraph delves into the reasons behind the Earth's seasonal changes, which are primarily due to its axial tilt of 23.5 degrees, rather than its distance from the Sun. It clarifies the misconception that seasons are caused by the Earth's elliptical orbit, emphasizing instead the angle at which sunlight strikes the Earth. The paragraph also touches on the precession of the Earth's axis, a slow wobble that takes 26,000 years to complete, which affects the position of the North Star and the dates of the Sun's location in the zodiac constellations. It concludes by reflecting on the importance of naked-eye astronomy and the deep connection ancient humans had with the stars, contrasting it with the modern detachment from the night sky due to urban light pollution.

Keywords

💡Polaris

Polaris, also known as the North Star, is a significant celestial object due to its unique position in the sky, nearly aligned with Earth's axis of rotation. In the script, Polaris is mentioned as a reference point that remains relatively fixed in the northern sky, serving as a navigational aid for observers. Its stability is contrasted with the gradual movement of other stars and constellations throughout the year.

💡Earth's Rotation

Earth's rotation refers to the spinning of the planet on its axis, which occurs approximately once every 24 hours. The script describes how this rotation causes the daily cycle of stars rising in the east and setting in the west, creating a familiar pattern in the night sky that repeats with minor variations from one night to the next.

💡Orbit

The term 'orbit' in the script refers to the path that Earth takes around the Sun, which is completed once every year. This annual orbit is responsible for the changing positions of stars and constellations as viewed from Earth, as well as the progression of seasons and the movement of the Sun along the ecliptic.

💡Ecliptic

The ecliptic is the apparent path of the Sun across the celestial sphere as observed from Earth. The script explains that the ecliptic is significant because it represents the plane in which Earth orbits the Sun, and it is along this line that the Sun appears to move through the zodiac constellations over the course of a year.

💡Zodiac

The zodiac, as mentioned in the script, is a circle of constellations that line the ecliptic and through which the Sun appears to move. The zodiac is divided into twelve parts, each represented by a constellation, and is used in astrology to describe the position of celestial bodies at a person's birth.

💡Precession

Precession, as described in the script, is the slow, conical motion of Earth's rotational axis, which takes about 26,000 years to complete one full cycle. This phenomenon causes the orientation of Earth's axis to change gradually over time, affecting the position of the North Star and the dates when the Sun is in different zodiac constellations.

💡Seasons

Seasons are the result of Earth's axial tilt and its orbit around the Sun. The script explains that the tilt causes the Sun to appear at different heights in the sky at different times of the year, leading to variations in temperature and daylight hours, which we experience as the changing of the seasons.

💡Axial Tilt

Axial tilt, or obliquity, is the angle between Earth's rotational axis and its orbital plane. The script mentions that Earth's axial tilt of 23.5 degrees is what causes the variation in the Sun's path across the sky, leading to the different lengths of day and night and the occurrence of seasons.

💡Planetary Motion

Planetary motion, as discussed in the script, refers to the movement of planets like Mercury, Venus, and Mars around the Sun. The planets, which also orbit in the same plane as Earth, appear to change their positions relative to the Sun and stars over time, with inner planets showing noticeable motion over a single night and outer planets moving more slowly.

💡Naked Eye Astronomy

Naked eye astronomy is the practice of observing celestial objects without the aid of optical instruments. The script highlights the importance of this practice in human history, as it allowed ancient people to track time and seasons by observing the stars and planets, and it emphasizes the value of maintaining a connection with the night sky.

Highlights

The Earth's rotation makes stars appear to rise and set daily.

Observing the sky over multiple nights reveals subtle changes in star positions.

The Earth's orbit around the Sun causes stars to shift positions relative to the Sun over time.

Stars in the east rise about four minutes earlier each night due to Earth's orbit.

The ecliptic is the apparent path the Sun takes through the sky, reflecting Earth's orbit.

The zodiac constellations are those the Sun passes through in a year.

Planetary motion is observable as they orbit the Sun in the same plane as Earth.

Inner planets like Mercury and Venus show noticeable motion over a single night.

The Earth's axial tilt of 23.5 degrees causes the seasons.

In summer, the Sun's higher path in the sky results in longer days.

The Earth's tilt also affects the angle and efficiency of the Sun's warming, causing winter.

The Earth's orbit is elliptical, but it's the angle of the Sun's rays, not distance, that causes seasons.

Seasons in the northern and southern hemispheres are opposite due to Earth's tilt.

The Earth's axis precesses, changing the celestial pole star over thousands of years.

Precession also alters the date the Sun is in a particular zodiac constellation.

Astrology's inaccuracies are partly due to precession's effects over 2000 years.

Naked eye astronomy was crucial for ancient humans, serving as a natural clock and calendar.

Modern knowledge of the sky is a result of centuries of observation and scientific application.

The loss of connection with the night sky is a consequence of urban light pollution.

Encouragement to appreciate the Universe by observing the stars and understanding their significance.

Transcripts

Under a dark sky, you can see thousands of stars. If you watch for a few hours, you can

see them rise and set as the Earth rotates once a day.

And if you go outside the next night at the same time, you’ll see that things’ll look

pretty much the same as they did the night before. The stars rise and set, Polaris hangs

to the north, and so on.

One day hardly makes any difference to the sky’s appearance.

But what if you wait for another night? Or a week? If you’re that patient, and observant,

you can spot subtle changes in the sky.

Let’s say a couple of weeks have passed. Remember that star that was just over a tree

in the east when the Sun set -- the one that made you first notice the stars are rising

and setting? Go look at it again.

If you happen to be out at the same time, you’d expect that star to be in the same

place. But it’s not. It’s actually a bit higher above the tree. And if you look west,

stars that were well off the horizon just after sunset last week are now lower.

If you wait a month, this effect is even more pronounced; all new constellations will be

visible in the sky after sunset. This is because the Earth is going around the Sun, literally

changing our viewpoint on the sky.

The Earth takes a year to orbit the Sun once. Every day, it moves a little bit along its

orbit. And as it does, from Earth’s perspective, distant stars appear to move their positions

relative to the sun. So, one day we might see a star very near the Sun, but the next

day the angle is a bit bigger.

At some point, about six months after we first saw it, the star is directly opposite the

Sun in the sky. Then the angle starts to shrink again as the star approaches the Sun from

the opposite side, until, after a full year, the cycle repeats.

What this means to you, the naked eye observer, is that the stars appear to rise and set at

different times over the course of the year. Stars in the east rise about four minutes

earlier every night, and stars in the west set four minutes earlier. A constellation

that was entirely below the eastern horizon at sunset one month might be completely visible

after sunset the next month.

Another way to think about it is that the stars appear to be fixed, and as the Earth

circles the Sun, the Sun moves through the stars over the course of the year, making

a complete circle around the sky once per year.

The path it takes is a reflection of the Earth’s path around the Sun, a line in the sky. We

call that line the ecliptic.

That means the Sun passes through the same constellations in the sky every year. We give

those constellations a special name: the zodiac. Every year, during a given month, the Sun

will appear to be in a certain zodiacal constellation, from Sagittarius through Scorpius, Libra,

Virgo, Leo, Cancer, and the rest.

Eventually, over a year, the Sun returns to Sagittarius, and the cycle starts again. But

even though we talk about this process in terms of the sun’s movement, it’s really

the path traveled by the Earth that creates this effect, as our perspective moves with it.

And of course, the planets move in the sky as well. Mercury, Venus, Mars… they orbit

the Sun, too, and they do so in approximately the same plane the Earth does. If you could

see the solar system from the side, it would look flat!

So to us, on Earth, the planets go around the sky over the course of a year, and they

also appear to change their positions relative to the Sun and the stars.

The inner planets, Mercury and Venus, move so rapidly you can see their motion after

a single night. The outer planets are more leisurely, but wait long enough and they too

will be seen to move, sliding through the constellations.

By the way, the word “planet” is Greek for “wanderer."

There’s another aspect of all this you might notice over time.

You’ve probably seen a globe, and noticed that the axis of it is tilted; that is,

it’s not straight up-and-down, perpendicular to how it sits. That’s because a globe is

modeling the Earth and the Earth is tilted.

The Earth spins on its axis once per day, and orbits the Sun once per year. But the

Earth’s axis is tilted with respect to its orbital plane by 23.5 degrees. And this has

a profound effect on our planet.

Imagine for a moment that the Earth’s axis were exactly perpendicular to its orbit, straight

up and down. If that were the case, every day, the Sun would take the same path across

the sky. If you were on the equator the Sun would rise, go exactly overhead, and then

set. If you’re on the pole, the Sun will appear to go around the horizon every day,

neither rising nor setting — it would always be twilight.

But that’s not the case. The Earth is tilted. In the months of June and July, the Earth’s

north pole is tipped toward the Sun. Six months later it’s pointed away. This affects the

path the Sun takes across our sky.

Instead of it taking the same path every day, in the northern summer, when we’re tipped

toward the Sun, the Sun takes a higher path in the sky. Because that path is longer,

the days are longer, too.

Six months later, in December and January, the Earth’s pole is tipped away. The Sun

takes a lower path in the sky, and because the path is shorter days are shorter too.

That’s why we have seasons! When the Sun is up high in the sky it shines straight down

on the ground, heating it better, and days are longer so it has more time to heat us

up. It gets hot.

In the winter, it’s the reverse: The Sun is lower so it can’t warm us up as efficiently,

and it has less time to do so. It gets cold.

There you go: seasons. The Earth’s axis is tipped. If it weren’t, the seasons wouldn’t

occur, and the temperature of the Earth wouldn’t change month to month.

There’s a common misconception that the Earth has seasons because it orbits the Sun

on an ellipse, and so it’s closer to the Sun in summer and farther in winter. While

it’s true the orbit is elliptical, Earth is closer to the sun in January -- on the

order of 5 million kilometers or so -- than it is in July. It’s the angle of the sun’s

rays that makes winter cold and summer hot, not our distance from the sun.

Also, you may know that when it’s summer in the northern hemisphere, it’s winter

in the southern. When the north pole is tipped toward the Sun, the south pole is tipped away,

so northern and southern hemisphere seasons are opposite each other.

But nothing in astronomy is permanent. The north pole’s not always going to point toward

the sun in June, and Polaris is not always going to be the North Star.

That’s because our planet’s axis is actually moving.

Have you ever seen a spinning top start to wobble, its axis moving in a slow circle even

as the top itself spins? This is called precession, and the Earth does it too! Our planet spins

on its axis once per day, but the axis wobbles, making a very slow circle that takes

26,000 years to complete.

This affects a lot of what we see in the sky. For example, Polaris won’t always be the

pole star! Every year, the pole points a little farther from that star, making a big circle

47 degrees across. For ancient Egyptians, the star Thuban was the pole star, and in

about 11,000 years that position will be held by the bright star Vega.

Also, the date the Sun is in a particular zodiac constellation changes slowly due to

precession as well. When the ancients first thought up this idea, the Sun was in Aries

on March 22, the vernal equinox (what some people call the first day of spring). But

due to precession, it’s now in Pisces! That’s why your astrological sign doesn’t match

where the Sun actually is in the sky; 2000 years of precession has changed them…one

of the many reasons astrology is silly.

It’s incredible to think about: The Earth, the Sun, the stars: they allow us to tell

the time and time of year just by looking up and paying attention. This is why the stars

were so important to ancient humans. The stars were like a clock and a calendar in the sky,

long before we had invented either.

We’ve actually learned a lot about the sky just by looking at it. Of course, some of

the stuff I’ve explained we’ve learned through other means – the Earth is spinning,

stars have different intrinsic brightnesses, and so on. But all of that knowledge, and

far more, got its start by people who went outside and looked up. Later, as we applied

math and physics to what we observed we learned even more, and could then go back

and explain what we saw.

So don’t discount naked eye astronomy; it’s all we had for thousands of years.

In fact, I think we lost something when we started using clocks and calendars, and moving

to cities with bright lights that washed away the stars from the sky. Those folks long ago

were tied to the sky; they knew it like you know the streets in your neighborhood. They

could see the stars rise and set, they knew the glory of the Milky Way sprawled across

the heavens, even if they didn’t know exactly what it was.

We do know, now, with our knowledge gained over the centuries. But it comes at the cost

of losing touch with the sky, not living under it as much as we once did. I’ve spent thousands

of hours over my life at night just simply looking up, watching the stars, appreciating

the Universe as I can see it. The things I have witnessed have shaped my life, and instilled

in me a permanent and endless sense of wonder and joy.

The Universe belongs to everyone. Go outside and, if you can, soak up your share.

Today we talked about cycles: As the Earth goes around the Sun we see stars rising and

setting at different times, the Sun moves along a line in the sky called the ecliptic,

through a set of constellations called the zodiac — really a reflection of the Earth’s

motion around the Sun — that the planets move more or less along the ecliptic as well,

and that seasons are caused by the tilt of the Earth’s axis together with its annual

orbit around the Sun.

Crash Course is produced in association with PBS Digital Studios. This episode was written

by me, Phil Plait. The script was edited by Blake de Pastino, and our consultant is Dr.

Michelle Thaller. It was co-directed by Nicholas Jenkins and Michael Aranda,

and the graphics team is Thought Café.