On April 20th (4/20) at midnight Earth’s horizon (in northern latitudes) lines up perfectly with the plane of the entire Milky Way galaxy. The galaxy “wraps around” our view of the sky and we can see into and through the plane of the galaxy.
At this moment when you look out around you at the horizon, you are looking into the plane of the Milky Way galaxy.
How to See the Milky Way Galaxy Plane
Look southward (and a little bit east) and you are looking toward the center of the galaxy. Look northward (and a little bit west) and you are looking toward the outer edges of the galaxy.
Why does the galaxy line up with the horizon?
Each year on 4/20 the galaxy lines up with the horizon – an “event” I call the Galaxy Horizon. In fact, this “event” occurs every day! It’s just an interesting coincidence that the alignment happens on 4/20 at midnight. Each day the galaxy lines up again with the horizon but it happens 4 minutes earlier.
Earth Map in the Sky – Landforms as Constellations
Learn how to see the map of Earth in the starry sky.
Stars help us find our way. Stars are like a giant map in the sky that tells us where we are on the surface of the Earth. Sailors use them as a “map” to navigate the world. For thousands of years, the stars were stationary markers of latitude and longitude.
We are going to learn to map something new onto the sky: locations on the Earth! We can create an exciting new set of “constellations” out of the shapes of the continents on the Earth.
We live on a sphere so we can see half of the sky (a hemisphere) at any one moment. It’s easy to imagine half the Earth mapped onto half the sky. Keep reading to learn how.
Wherever you are on the Earth, when you look straight up (toward your zenith), you might see one star, but there are a bunch of other stars within view. All of the stars you see in the sky are directly overhead some other place on the Earth. Every place on Earth has their own set of stars directly overhead – their “zenith stars.”
Look up at any star in the night sky; that star is directly over some place on Earth. There are hundreds of “faraway zeniths” up there.
World Zeniths – See the Map of the Earth in the Sky
Every star maps to a location on Earth and every location on Earth maps to a star.
If you live in the western hemisphere, you can learn to look up and “see” the land borders of the North American and South American continents visible, projected into the sky like a giant painting on a curved ceiling. You can learn to see even more landforms in the sky – you can learn to see the entire western half of the Earth projected in the sky.
Visualize Countries in the Sky
We can learn to see country outlines in the sky. The key is to imagine yourself at the center of the Earth looking out into space “through” a translucent Earth surface.
Here is a good way to visualize these countries-in-the-sky even when you are on the surface. Imagine that you can look up and see your location at the zenith.
When I do this, I see southern New York state, Long Island jutting out into the water like a long pier, and the wide Hudson River emptying past New York City. Eastward is the dark expanse of the Atlantic Ocean and low on the eastern horizon are the countries of Europe and West Africa.
Westward in the sky, I can see the outline of the west coast of the US. Then, there is a big blank space of the Pacific Ocean and a spot near the western horizon that is Hawaii.
The Map of the Earth in the Sky is Reversed
There is one odd thing about the map as you see it in the sky… it’s reversed – as if seen in a mirror! This happens because we project the map lines outward into space toward the stars. When we look at the map this way it’s as if we are “inside” the Earth looking outward.
The map of the USA covers about 58˚ of sky from east-to-west. 58˚ is about 2x pinky-to-thumb (spread out all your fingers of both hands and touch thumbs). Your left pinky tip should be on your zenith. If you are in New York or somewhere on the east coast, the right pinky tip will indicate the approximate western edge of the USA.
Physical Astronomy – Stars Map to Places on Earth
Learn to see the zenith map in your sky using this Physical Astronomy technique.
Exercise 1: face south and point high in the sky.
Face south. Then, reach both hands straight up over your head and point above your head with both pointer fingers. You are pointing at your zenith. Now, bring both arms down until they are pointing one due east and one due west. You are pointing at two points in the sky that are zeniths for someone else.
Secret! You Can See a Star That Another Person Can’t
If you do this physical astronomy exercise right after sunset, the eastern and southern zenith locations are in night, but the western and northern sky points are over Earth locations that still have daytime.
This means that you can see the star that is at their zenith, but they cannot see that star. For example, Seattle still has 3 hours of sunlight left in their day so stars are invisible behind blue sky. The city of Yekaterinburg is on the opposite side of the world and just after sunset in New York it faces the Sun and has a bright daytime sky!
We are on the night time side of the Earth and we can see the current zenith stars of Seattle and Yekaterinburg – but people who live in these cities cannot see them! They have to wait to rotate to the night time side of the Earth to see stars.
The Math – How High Up is the Zenith Map?
Project an imaginary map of the Earth into the sky. The map has to be the correct size so that when it is viewed from a distance it “covers” the same distances.
If a map is too close, it is just the same size as the territory. So, we have to choose the correct distance to project the zeniths. As the zenith map “projector screen” moves away from the Earth we see more of the borders of the Earth. But, at some point the distance of the map corresponds exactly to the faraway zeniths.
Our question is: “How far away from the Earth do you have to be so the landforms (like the continents) have an angular diameter that is equivalent to their “actual size” in the sky?” How far away does our imaginary zenith map USA (about 3000 miles wide) image have to be to cover 58 degrees of arc in the sky?
To answer this we need math.
The Zenith Map Distance from Earth
The Earth is approximately 24,901 miles in circumference at the equator. If we can see half the sky from any point on the Earth, then we can “see” half the Earth projected onto the sky by the zenith map. That means that for 180˚ of sky we can “see” about 12,450 miles of the Earth’s surface projected into space. 12,450/180 = 69 miles. When 1 degree of arc spans 69 equatorial miles the image is “at” the correct distance.
1 Degree of Sky equals 69 Miles
So, at the equator every degree of sky covers about 69 miles in every direction. As you go towards the poles the longitude degrees (east and west) cover less and less zenith map distance, but the latitude degrees (north and south) always stretch 69 miles. Every 15˚ of sky equals about 1035 (69*15) miles.
The distance between your pointer finger and your pinky (when you hold your arm and hand stretched out in front of you) is 15˚ – so you are measuring about 1035 miles on Earth with that sky measurement. One pinky width is equal to 1˚, which is 69 miles of zenith map!
The Math – Inverse Tangent and Angular Diameter
There is a simple calculation that helps us determine how far away something needs to be to fill just 1˚ of the sky. Here we use just a tiny drop of trigonometry to discover the “tangent of 1 degree.”
The tangent of 1˚ is 0.017455. The inverse of something is when you divide 1 by the number you want to invert. So, the inverse of 0.017455 (1/0.017455) is 57.29. The inverse of the tangent of 1˚ helps us figure out the distance something has to be to appear to be 1 degree angular diameter.
So, 57.29 * 69 miles = 3,953 miles away! This is how far away the “map” has to be to show you your hemisphere of the Earth map. 3,953 miles is higher than low Earth orbit (LEO) satellites (lower than 1200 miles); it’s closer than geosynchronous satellites (at about 23,000 miles); and it’s about 1/60 the way to the Moon.
So, imagine that the Earth map is projected onto a screen – an imaginary celestial sphere, shell-shaped – that is quite close to the Earth and encircles us. It shows us our Earthen landforms and the oceans beside, superimposed in the sky.
We live on a sphere. When we look at out night sky we are able to see stars low on our horizon that are visible directly above someone else – one-quarter the way around the around the world in all directions.
If you live within 6 time zones of someone that means that you share some “simultaneous sky.” Anyone living further than 6 time zones away sees a completely different sky – unless you can see circumpolar stars that dip under the North Star. That means that you can see countries past the North Pole and down the other side of the globe.
Your zenith is yours – it is unique and changing all the time. Not even someone standing right beside you shares your zenith. You can use this idea of the zenith stars to comprehend the vast and mysterious experience of life on a sphere.
Many people ask “Why can’t I feel the Earth spinning?” Or “Why don’t we feel the rotation of the Earth?” The answer is that the Earth is so large and we are so small in comparison that we just get carried along for the ride.
We humans are tiny so we don’t notice that everything on Earth moves along with us. Until we look up and we see the Sun speeding through the sky – then the questions start to fly: “Why does the Sun move across the sky?”, “How big is the Earth?”, “How fast is the Earth spinning?”
It’s hard to imagine just how large the Earth is and just how fast it is carrying us around in its daily motion. How fast is the earth spinning?
You are about to learn the answer and learn how to experience this speed!
We are too small to see the illusion
The Earth spins at about 1000 mph (1609 kph) at the equator. But we can’t feel it. This illusion of non-motion happens for several reasons:
Because the Earth is enormous compared to us, and
Everything around us moves in the same direction and speed, and
Our inner ear is not sensitive enough to feel the gradual changes in momentum caused by Earth’s constant turning.
Imagine riding in a fast car that has no windows. The car cruises on a smooth highway at 100 kph (62 mph). Inside the car we chat with our friends, the seats are comfortable, and maybe a hot drink sits placid in a cup holder. We don’t feel the motion at all unless the car turns or speeds up or slows down.
Everything seems to be staying still but this stillness is an illusion. All of this stuff in the car is moving along with us at the same speed. But, if you could open a window in the car you would see the motion of the other objects not in the car.
The sky is our open window to find those other objects that are not “on the Earth.” We will use them to explore the Earth’s speed and movement.
Motion senses don’t feel the motion
Another reason that we can’t feel the Earth spinning is because our sense of balance. We get this sense from organs of the inner ear. We know what it feels like to move. We can feel a car accelerate and turn – but these sense organs don’t help us feel the Earth’s changes in speed and constant turning.
It’s easy to feel and then believe that the Earth stays still and everything else moves around us; the Earth doesn’t seem to be moving. The idea of a moving Earth seems preposterous. It’s no wonder it took humans so long to figure out that the Earth is spinning and moving!
However, you can learn a surprising way to feel the rotation of the Earth – and it has to do with the speed of sound. You can “feel the Earth spin under your feet” and all it takes is a little bit of imagination.
Speed of sound is the speed of day
The speed of sound in air is 1235 kph (767 mph). This is really fast, but it is still a visible and imaginable speed.
The speed of the Earth’s rotation at the equator is faster than the speed of sound! But, because of the Earth’s shape, the Earth’s rotation speed is exactly equal to the speed of sound at two latitudes on the Earth: 42.97 North and 42.97 South.
What does this mean for you and your approach to physical astronomy? You have to train your brain to see the speed of sound, then you will be able to feel the Earth spinning. You will understand that the speed of sound is the speed of day.
7 ways to see the speed of sound
We can train our brains to “see” the speed of the Earth’s rotation (I’ve named this speed the “Speed of Day“). This is possible because “the speed of sound = the speed of Earth spinning.”
The fastest plane you’ve ever seen is traveling a bit slower than the speed of sound. Imagine the fastest jet plane you have ever seen. Unless you were watching a fighter jet, that jet was not going at the speed of sound. Commercial jet planes travel about 250-350 mph below the speed of sound.
Echoes, echoes, echoes… The speed of sound can be heard and easily visualized by exploring sound echoes that bounce off walls, fences, hillsides, and other objects.
Large fields. Go to a large field (as big as a football field or bigger) with a friend and ask them to clap. You will see your friend’s hands clapping and then hear the clap.
Stadium concerts. You can experience the speed of sound at big stadium concerts where you can see the drummer hit the drums but the sound is not in synch with what you see.
Lightning and Thunder. You can feel the speed of sound when you see the flash of distant lightning and you don’t hear the boom of thunder for several seconds. Every 5 seconds is about 1 mile.
Fireworks. You can witness the speed of sound when you are watching fireworks and you hear the bang of the explosion after you see the light of the explosion.
Videos. Finally, there are several good videos online that can help you visualize the speed of sound (links below – includes a clever piece from the Exploratorium in San Francisco).
The speed of rotation of the Earth
How fast is the Earth spinning? Once you have a clear idea of how fast sound goes, you can get a clear idea of how fast the Earth is spinning – and by experiencing this speed you can easily imagine just how incredibly large the Earth is.
A gigantic bell rings, “Races the day”
One way of visualizing the speed of the Earth’s rotation is to imagine that a gigantic bell rings each time the Sun enters a new time zone. Imagine that there is no sound loss over distance and the sound of that bell can be heard across the whole time zone (up to 1000 miles).
When this bell rings, the sound takes time to travel. It will travel at the speed of sound: 767 miles per hour.
Sound = 767 miles / hour
Bangor, ME and Detroit, MI – two American cities on opposite ends of the eastern US timezone – are 1183 km (735 miles) apart. The sound of our imaginary bell takes about 1 hour to travel between these cities.
Earth surface = 767 miles / hour
Since these cities are near 42.97 N latitude, the Earth takes about 1 hour to spin underneath the Sun.
Sound, which seems quite fast, is actually slow enough to “see” it moving. Once you can see the speed of sound you have seen the speed of the surface of the Earth.
The Earth rotates once per day. People at the equator are moving at faster than the speed of sound, people near the poles are moving slower than the speed of sound. People living at latitude 42.97 N and 42.97 S are moving at exactly the speed of sound.
Editor’s Note: Credit goes to Bob Berman (aka SkyManBob) for first pointing out the latitude/speed of sound correspondence to me. I first learned about this idea in his fantastically engaging book Zoom – How Everything Moves. It has a footnote in Chapter 5 that says people at Coney Island are moving at 795 mph – just over the speed of sound. It got me wondering… how can people experience this speed? The result is this article. Thanks, Bob!
Editor’s Note: Another Bob (aka AstroBob) gets credit for helping me refine this article with insights about our common perception of motion.
You can see Venus in the sky at two times and locations:
in the early evening, shortly after sunset in the west or
the early morning, shortly before sunrise in the east.
Venus orbits the Sun and moves from evening sky to morning sky and back again over the course of about 18 months. Venus makes beautiful sweeping motions in the sky that reveal secrets of the solar system.
The orbits are all in the same plane. It’s like they are all marbles circling around the sun on the same giant plate. This is called the “ecliptic” and it is visible in the sky if you know how to find Venus.
Intersecting space planes
The “space plane” is not an airplane
The “plane” is a tool you can use to see the way things move in space. This “plane” is not an airplane, but a flat slice of space.
Here is an image of two intersecting planes. Imagine the blue plane is the earth’s surface and the brown plane is up-and-down from ground to sky.
Each object (and movement) in space creates a “plane,” an imaginary slice through physical space. The blue “plane” above looks like the surface of a pond, lake or ocean. A wall or roof of a house is a plane. A dinner plate is a plane. Stretch your arms out and spin in a circle and you have created a plane with your arms.
There are planes in space everywhere.
Your own personal space plane
You create a plane with your vision and balance. You can imagine a flat surface like the surface of a pool of water and your eyes are just above the waterline. This surface moves and tilts when you move your head.
Your head has two eyes that define your plane of vision. Also, your body is oriented to gravity because of your sense of balance – the “personal horizon” is the first plane for you to orient to. Your body naturally coordinates your visual sense with your sense of balance and gives us the sense of being located level on a surface. This is the “sense of horizon.”
A new horizon – choose a plane!
To get good at Physical Astronomy, we have to learn to coordinate our main “personal horizon” plane with other planes of the earth, moon, solar system, galaxy, and universe.
The earth for instance, has a lot of planes, the range of latitudes, the north and south poles, the Arctic and Antarctic circles, the equator, the tropics, a range of longitudes, the prime meridian, the international date line, the ecliptic, the galactic plane and more.
To keep things simple, let’s focus on just one other plane for now: the plane of the solar system. A wonderful thing will happen when you learn to link the plane of vision with the plane of the solar system. It’s pretty easy to do, and it’s a skill that gets better with practice.
The key to linking vision and solar system planes is to know that the plane of the solar system is visible as the ecliptic. One easy way to see the plane of the solar system is to see the bright inner planet Venus.
See Venus and the Orbit of Venus
Venus is closer to the Sun so we are able to see its entire orbit. Actually, we can’t quite see the entire orbit because sometimes it goes in front of the sun and sometimes it goes behind the sun.
We can see Venus in the early evening and in the early morning. Venus is visible in our sky when it is at the left and right extent of its orbit around the sun. We only see Venus in the sky when it is swooping around the left or right of the sun.
If you want to see the orbit of Venus and see the plane of the solar system you can do it! All you have to do is imagine a line connecting Venus to the Sun.
If you are looking at Venus early in the morning before sunrise or early in the evening before sunset, the process is the same. Imagine a line connecting Venus to the Sun; this is the ecliptic. Venus’s entire orbit covers roughly 1/4 of the sky.
We learned about two main planes in space: your personal horizon (which changes as the Earth carries you around the Sun) and the plane of the solar system: the ecliptic.
By learning to visualize these two space planes, we can begin to experience the extremely large dimensions of space.
One Day of Observation: the Sun rises and the Sun sets
Let’s start with a few easy observations about daytime. These are things you can notice just by waking up early one day before the Sun brightens the night.
The Sun starts the day for us on one side of the sky and ends the day on another side. At both of these times (sunrise and sunset), the Sun appears near to the ground – at the horizon.
During the middle of the day, the Sun appears to move “up” and across the sky and then back “down” again. In the middle of the day – at noon time – the Sun is high up in the sky, away from the ground.
Shadows change during the day
In the morning the Sun makes long shadows. At noon the Sun makes short shadows. At the end of the day, the Sun makes long shadows again.
With a few simple tools you can measure the Sun’s position and shadows.
The Sun moves east to west
Over the course of one day, the Sun appears to move across the sky from east to west, rising to the highest point at noon. The Sun’s light shines on the Earth and makes shadows that move and change position and size. As the Sun “moves” through the sky, the shadows move on the ground.
Build a simple sundial, track the Sun
A sundial tracks the shadow of the Sun with an object that casts a shadow and time markings. For the simple sundial you can use a stick. The shadow of the stick (the stick on a sundial is called a gnomon) moves across the sundial. The shadow of the stick points to the time markings.
The simplest sundial is just a stick stuck in the ground with time markers nearby. The location of the stick’s shadow moves across the time markers throughout the day.
Mark the shadow’s position with any object (chalk drawing, a rock or another stick is a good choice). In the morning, the Sun appears low in the east and the shadow is long. The morning shadow points toward the west. At midday (noon) the Sun is at the highest point so the shadow falls in the middle and becomes short. At sunset, the shadow becomes long again – pointing to the east.
Paper Plate Sundial
A paper plate with a pencil stuck through the middle makes a great moveable sundial! (Remember, when you move a paper plate sundial, you have to be careful to place it in perfect north-south alignment.)
Take this outside on a sunny day, then make a mark on the paper plate at the top of each hour. The shadow will move slightly each hour. The mark should go at the middle of the pencil shadow.
When you have completed this during one sunny day, you have made a sundial that can tell the time – roughly speaking!
An indoor sundial – the Sun Tracker
Most people think of sundials as something that you place outside. But, the Sun shines inside through windows. You can track the Sun through a window.
An indoor sundial can help you track the Sun from the comfort of your own home! Do you have a sunny (or partly sunny) window? You can track the Sun and reveal the secrets of the Earth’s motion.
The Sun Tracker is an easy-to-use indoor sundial. Place the glistening window cling on any sunny window and then mark the position of the window cling’s shadow using one of the included stickers.
Repeat the next day or the next week at the same time of day. You will see a pattern emerging: the shadow cast by the Sun moves quite a bit each day.
If you are extra precise with recording the shadow at the same time of day, and you are able to do it for an entire year… you will see the Analemma.
The Sun Tracker is like a little bit of Stonehenge for your window.
Track the Sun with simple tools and you will reveal the motion of the Earth. There are two main motions of Earth, daily rotation and yearly orbit. Earth spins under the Sun each day and around the Sun in an orbit each year.
It is these two motions that make the Sun seem to move in the sky. Remember that the next time you are looking for the Sun – it’s where it always is… the Earth is what moves.
The Earth is like a merry-go-round showing us seasonal constellations
That iconic childhood ride. Round and round each day we go, round and round each year we go, where we stop nobody knows! When we look out from the edge of the ride we can see the space beyond. Sometimes the Sun occupies that space, and sometimes that space is the night sky filled with stars.Click here to continue reading…
Your hands and arms help you see the orbits of Mercury and Venus and the shape of solar system
Question: If you could see the orbit of Venus would it fill the whole sky?
The answer might surprise you!
You can use your hands and arms to see the size of the orbits of the solar system’s inner planets: Mercury and Venus.
Imagine (as pictured below) if the orbit of Mercury were visible as a red oval and the orbit of Venus were visible in green.
Physical Astronomy – see Mercury and Venus orbits
Caution! Do not look directly at the Sun without proper solar safety glasses on.
Turn toward the Sun, hold your arms out straight, hands up in the air with fingers spread wide and thumbs touching. Your pinky fingers now span the width of the orbit of Mercury and your elbows span the width of the orbit of Venus.
Both of the entire orbits of Mercury and Venus orbits would be visible in the sky all at once – if they could be made visible during the day.