ASTR 1P01 - Lecture 2: Looking Up to the Sky Summary

Lecture 2: Looking Up to the Sky

Goals

Understand how astronomers describe the sky.
Learn about constellations.
Discuss the major celestial bodies seen with the naked eye.

The Celestial Sphere

The celestial sphere is an imaginary sphere used to visualize the sky.
It is not a real sphere.

The Celestial Dome

Wherever you are on Earth, half of the celestial sphere is above you, forming a celestial dome.
Zenith: The top of the dome, directly above you.
Horizon: The place where the dome seems to meet the ground.

The Horizon

The horizon is a circle because the dome is round.
The full circle is rarely seen due to obstructions (buildings, mountains) or poor visibility.

The Celestial Poles

Imagine a line going through the Earth, connecting the north and south poles, this is the axis of rotation of the Earth.
Extend this line until it meets the celestial sphere. The intersection points are the north and south celestial poles.
Looking up from Earth’s north pole, the sky rotates counter-clockwise.
Looking down from the north celestial pole, the Earth rotates counter-clockwise.

The Rotation of the Earth

The Earth completes a full rotation approximately every 24 hours.
This rotation causes the rising and setting of the Sun, Moon, and stars.
They all rise in the east and set in the west.
The Earth rotates from west to east.

The Day and Night Cycle

The Earth's rotation is also responsible for the day and night cycle.

The Celestial Equator

The equator is an imaginary circle halfway between the north and south poles.
Extend this circle out towards the celestial sphere to get the celestial equator.

Angles

There are $360^{\circ}$ in a circle.
Half a circle is $180^{\circ}$ .
A quarter of a circle is $90^{\circ}$ .

Latitude and Longitude

Latitude: The angle a point on Earth's surface makes with the equator, either north or south.
- The latitude of the equator is $0^{\circ}$ .
- The latitude of the north pole is $90^{\circ} N$ .
- The latitude of the south pole is $90^{\circ} S$ .
Longitude: The angle a point on Earth's surface makes with the prime meridian.
- The prime meridian is a line stretching from pole to pole through the Royal Observatory in Greenwich, England.
- The prime meridian has a longitude of $0^{\circ}$ .

Latitude and Longitude Mnemonic

Switch the first two letters of "latitude" to get "altitude", another word for "height".
Maps of the Earth always have north on top, so the latitude tells you the “height” along this map.

Star Circles at the North Pole

At latitude $90^{\circ} N$ , the north celestial pole is at your zenith, and the celestial equator is at your horizon.
You can only see half of the sky, and always the same stars.
The stars don't rise or set; they circle around the pole.

Star Circles at the South Pole

The center of these circles is the south celestial pole.
Because the Earth rotates around the pole, the pole itself doesn't change its position in the sky.

Star Circles at the Equator

At latitude $0^{\circ}$ , the celestial equator passes through your zenith, and the celestial poles are on your horizon.
Over a 24-hour period, you will see all the stars in the sky rising in the east and setting in the west.

Star Circles at Niagara Falls

In Niagara Falls, $43^{\circ} N$ , the north celestial pole will be $43^{\circ}$ above the northern horizon.
The south celestial pole will be $43^{\circ}$ below the southern horizon, so it will never be visible.
Stars within $43^{\circ}$ of the north pole can never set. They are always above the horizon and circle around the north celestial pole. This part of the sky is called the north circumpolar zone.

Patterns in the Sky

On a perfect night, it is possible to see about 3,000 stars with the naked eye.
The positions of these stars are random, but humans find patterns in them.
Ancient cultures named specific geometric patterns that were particularly noticeable.
These patterns of stars are called asterisms.
Identifying these patterns is like solving a celestial connect-the-dots puzzle.

Asterisms and Navigation

Historically, asterisms were helpful in navigation, allowing people to orient themselves at night.
Here are some examples of asterisms:
- The Big Dipper: Consists of 7 bright stars that look like a bowl with a handle.
- The Little Dipper / Ursa Minor / Little Bear: Similar in shape but smaller in size. The bright star at the end of the handle is Polaris, the North Star, which is very close to the north celestial pole.

Constellations

In modern times, astronomers divide the celestial sphere into 88 regions, called constellations.
These regions cover the entire sphere, so any point in the sky is located in exactly one of the constellations.
Each constellation contains one or more prominent asterisms.
Constellations get their names from animals, objects, and mythological characters or creatures.

Historical Origins of Constellations

Many constellations originated as early as 5,000 years ago in ancient Mesopotamia (modern Iraq).
The civilizations of that area include the Sumerians and the Babylonians.
The ancient Greeks adopted the Babylonian constellations around 400 BC.
The ancient astronomer Ptolemy described 48 of the 88 modern constellations in his book Almagest around the year 150.
European explorers added new constellations that can only be seen in the southern sky during the 15th century.
The modern list of 88 constellations along with their boundaries was adopted by the International Astronomical Union in 1928.

Asterism vs. Constellation

"Constellation" means a region in the sky.
"Asterism" means a specific pattern of stars.
Each constellation has one or more defining patterns within its region, sometimes referred to colloquially as "the constellation".
The International Astronomical Union only defined the regions of each constellations, they didn't define how to connect the stars.
The Big Dipper asterism is located inside the constellation Ursa Major / Great Bear.
Constellation Orion is easy to find due to its distinctive asterism, Orion’s belt.

The Stars in 3 Dimensions

The sky looks like a 2-dimensional sphere, the celestial sphere, with the stars located on the sphere itself.
Some ancient civilizations thought that this was an actual sphere rotating around the Earth.
In reality, it's the Earth that's rotating.
More importantly, the stars are not located on a 2-dimensional sphere, they are distributed in 3 dimensions all over the galaxy.
Our sky, with its specific patterns of stars like Orion or the Big Dipper, is unique to us.
Aliens on a distant planet will see a completely different sky with different patterns of stars.
They will see the same 3-dimensional distribution of stars from a different location in the galaxy.
Stars that seem to be close together in our 2D sky might be very far apart in the actual 3D space.
If two stars happen to both be in the same direction as seen from Earth, they will appear close together in the sky, even if in reality they are many light-years away from each other.

Orion's Belt

In Orion’s belt, Alnitak and Mintaka are both ~1,200 light-years from us.
Alnilam is 2,000 light-years from us, so it’s not actually close to the other two!

The Movement of the Stars

The stars are not fixed in place. Each star has its own independent motion in 3D. Different stars move in different speeds and directions.
Over tens of thousands of years, the patterns that we recognize now in the sky may not exist anymore.
For example, one star in a constellation may move north while another one moves east.
Prehistoric humans saw a different sky.

The Pleiades

The Pleiades (PLY-a-deez), also known as the Seven Sisters, is a star cluster 444 ly away.
They’re actually close to each other in 3D space, so will still be seen together from different places and times.

Observing Constellations

Next time you’re far away from the city lights at night, take a look at the sky and try to find all the constellations mentioned in the lecture.
The course website has links to apps that you can install on your phone for free and will show you the location of the constellations when you point your phone towards the sky.

The Sun

The Sun is the closest star to Earth, only about 8.3 light-minutes away.
This distance is called an astronomical unit (AU), and it is equal to around 150 million km.
The Sun appears much larger and brighter than other stars because it is so much closer to us.
The Sun is so bright that you can get serious eye damage just by looking directly at it for a few seconds!
During daytime, when the Sun is in the sky, we cannot see any other stars in the sky, because they're just not bright enough compared to the immense brightness of the Sun.
This is why we can only see the stars during the night, or during a solar eclipse.
The Sun is also bright enough to turn the color of the sky from black to blue.

The Earth’s Rotation

The Earth rotates around its own axis and around the Sun.
A day is the period during which the Earth completes one rotation around its axis, about 24 hours.
A year is the time it takes the Earth to complete a full revolution around the Sun, about 365 days.

The Celestial Tilt

The plane of Earth’s rotation around the Sun is the ecliptic.
The line perpendicular to the ecliptic intersects the celestial sphere at the ecliptic poles.
The plane of Earth’s rotation around its axis intersects the celestial sphere at the celestial equator.
The two planes of rotation differ by an axial tilt of $23.4^{\circ}$ .
The two intersections of the ecliptic and the celestial equator are called equinoxes.
The Sun is at the equinoxes around March 20 and September 23.

The Sun's Path in the Sky

The Sun appears to move along the sky during the year because actually, Earth moves.
There are 13 constellations on the ecliptic.
The Sun appears to move in a circle eastward, and comes back to its original spot after exactly a year.
There are $360^{\circ}$ in a circle, and around 365 days in a year, so the Sun appears to move ~ $1^{\circ}$ per day.

Other Celestial Bodies in the Sky

With the naked eye, we can see 5 planets: Mercury, Venus, Mars, Jupiter, Saturn.
The Sun, the Moon, and these 5 planets were known in ancient Greece as the “7 classical planets”.
The word "planet" means "wanderer" in ancient Greek. These 7 objects seem to wander around the sky, while the stars stay in place.
In modern terminology, the word "planet" only applies to large objects that orbit the Sun.
The Sun follows the ecliptic on the celestial sphere.
The paths of the Moon and the planets are close to the ecliptic, but not exactly on it.
The orbits of the planets around the Sun, and of the Moon around Earth, all lie on their own planes, but these planes turn out to be very close to the ecliptic plane.

The Zodiac

The Sun, Moon, and planets are always within 8- $9^{\circ}$ north or south of the ecliptic.
This “belt” is called the zodiac.
“Zodiac" means “circle of animals”. Many of the 13 constellations on the zodiac are named after animals.

Conclusions

This lecture focused on things that can be seen with the naked eye: stars, asterisms, and constellations.
We also learned about many imaginary points and lines in the sky, such as the zenith, celestial poles, ecliptic, equinoxes, and so on.
Reading: OpenStax astronomy, section 2.1.
Exercises: Practice questions are available in the textbook and on the course website.