Train Your Brain – The Speed of Day

Physical Astronomy By Daniel Cummings

The Speed of DayThe Speed of Day

The “speed of day” is the speed of Earth’s rotation: 1600 kilometers per hour (kph) which is about 1000 mph.

1600 kph is fast. Most people have never experienced anything moving at this speed. Commercial jet airplanes fly at 1000 kph (600 mph). However, it’s hard to understand that speed because we don’t ever see them flying at ground level. The speed of sound is 1200 kph – the Earth’s rotation is carrying us 400 kph faster than the speed of sound. That is incredibly fast!

By using our imagination and “seeing” the speed of day we can understand just how fast we are moving. And by understanding how fast we are moving, and feeling just how long a day is – even at such a breathtaking speed – we can really feel the size of the Earth.

Stop Time

Let’s start by stopping time.

Pretend that everything is stopped. The Earth has stopped rotating and orbiting. It might seem strange to stop time to see something move, but it helps us to get our visualization right.

Pretend that you stop time at noon and you are standing outside facing southward. The Sun is now stopped in the sky at exactly noon at the highest point in the sky.

The Noon Curtain

The speed of Day - Noon curtainSo far we haven’t imagined anything too weird. But, we are about to add something nobody has ever seen before. Imagine that there is a very thin (1 cm thick) north-south, glassy, shimmering curtain of light (as thin as a veil) that ripples down from the Sun in the sky. The thin curtain or veil of rippling light is oriented directly north and south.

Because it is noon, this “noon curtain” drops from the Sun directly above you. The curtain appears to originate high above the atmosphere and drops down through the sky right down onto your head. It stretches to the north and south as far as you can see.

You can step to the left (east) and the curtain is to your right. You can step to the right (west) and the curtain is to your left. You can see it reaching from the Sun down to the ground like a gigantic shining, glowing curtain of light.

This “noon curtain” helps us visualize the line of noon in the sky. Every point on the Earth passes through this curtain every day at noon.

And remember, what we call noon is the time of day when the part of Earth we are on passes “through” this curtain.

Time is still stopped right now so the noon curtain is not moving.

Time stopped to see noon curtain

Start Time

Let’s restart time, slowly, to see what happens to the curtain of light.

Still facing southward you see the giant curtain begin to slowly “move” toward the west (the right).  As you know, the curtain is not actually moving; it is us moving. As time returns to normal the curtain begins to “move” faster towards the west. Really, as the Earth rotates, you are carried further and further away from the noon curtain. The curtain is “moving” because of the newly resumed movement of the Earth. You see the curtain seem to travel away westward but it is actually us traveling away eastward.

Eventually, the sun will “set” – actually the Earth carries us until the Earth itself begins to block our view of the Sun – and the sun and the “noon curtain” will disappear. All we have to do is to speed up time until just before noon the next day.

Slow Time

This time, to experience the noon curtain, instead of stopping time we can just slow it down a lot. Let’s slow down time by 300 times so that as the noon curtain approaches us from the east (actually we are carried through it), the curtain floats in at a slow walking pace (about 1.5 meters per second).

Imagine the towering curtain of light floating towards you from the east. It is moving as slow as a person walks. It crosses past you and it continues westward. During this noon curtain passage time is slowed so the Earth’s rotation is also slowed by 300 times. This is a speed that seems like it might correspond to the “speed of day” – the day goes by so slowly and the movement of the Earth is barely perceptible.

However, the Earth rotates 300x faster than walking speed.

Normal Time

For the last visualization let’s imagine passing through the noon curtain at full speed. What would that look like?

1 kilometer every two seconds.

The noon curtain spreads across the sky and we speed under it.

See the speed of day with the Noon Curtain visualization

The Size of the Earth

The distance we move to meet and go through the noon curtain is immense. Every day, 24 hours a day, the Earth rotates, carrying us on its surface under the noon curtain.

As you get good at visualizing this transit of the noon curtain, you can begin to feel just how big the Earth is.

Imagine the curtain “moving” constantly at this high speed for the entire day. This is the speed of the Earth’s rotation. We are carried along under the day at the speed of Day.


460 meters per second. Every day. The speed of day.

Arms around the ecliptic

Learn to see the ecliptic

Physical Astronomy by Daniel Cummings

Ecliptic Arms

The sun follows the same path through the sky every day.

Sun up. Noon. Sun down.

The sun starts the day in the east in the morning, rises high in the sky at noon, and settles down again in the west for a nap at night.

See the ecliptic

Here is an exercise to discover that path – the ecliptic.

Continue reading “Arms around the ecliptic”

Train your Brain – See the shape of the solar system

Physical Astronomy by Daniel Cummings.

Train your Brain - See the solar system disk

See the solar system from Earth

Most people, when they look up at the night sky can easily see stars and identify some familiar groups of stars (asterisms and constellations). Some people can even find and name some planets – Venus, Jupiter, and Mars are all bright and easy to see.

But, there are many invisible wonders in the sky – and some of them can be seen without a telescope. In fact they are so big that a telescope is not the right tool to use; we have to use something even more powerful… imagination!

Using visualization and imagination, I am going to show you how to find and “see” a very large structure in our sky: the solar system disk itself.

Continue reading “Train your Brain – See the shape of the solar system”

Night is…

“‘Night’ is not what time it is… it is where you are.” – Daniel Cummings

Stars like ours – 9 bright “Life Stars”

Life Stars – Visible Exoplanets

Physical Astronomy by Daniel Cummings

Life Star in the night skyA “Life Star” is a visible star that could host life. This is a name I came up with in February 2017 (around the time of the TRAPPIST announcement) to describe visible stars with confirmed planets orbiting in the habitable zone. “Life Star” is easier to say and explain. I hope it catches on! Continue reading “Stars like ours – 9 bright “Life Stars””

Stars in Your Eyes

Graphic showing text and milky way galaxy of stars. Stars end their trips in your eyes

The light of a star…

…no matter how long it has been traveling through space, ends its journey in your eye. The light hits your retina and is transformed into thought. As long as you let that star light stream into your view, the star itself lands in you and settles its motion.

When I was 17 years old I went on a weekend retreat run by the Christian Brothers near Melbourne, Australia. It was a weekend of spiritual teaching, introspection and reflection.

Saturday night, one of the other boys at the retreat surprised me. As we stood outside under the stars, he spoke in that matter-of-fact way typical of Australians.

Continue reading “Stars in Your Eyes”

Groundhog Day Shadow Tracker

Physical Astronomy Tool by Daniel Cummings

Groundhog Day Shadow Tracker Lesson Plan
It’s Groundhog Day!

What better way to celebrate Groundhog Day than to Build your own Groundhog Day Shadow Tracker

Let’s do some Physical Astronomy. The experience will help you to understand the movement of the earth and sun through the seasons. You will build a scientific instrument that is also a fun garden decoration and you will be able to track the Groundhog’s Shadow all Spring!

Setup and Process

Start with this cutout pattern and attach it to a post made of metal, wood, or plastic.

Groundhog Day Shadow Tracker Cutout Pattern


  1. Foam core or construction paper
  2. Scissors or hobby knife – foam core
  3. Wooden or plastic stick 12-24″
  4. Tape or glue for connecting Groundhog
  5. Permanent Markers or weather proof paint for decorating (Groundhog will stay outside)

Help the Groundhog find its shadow

On a sunny day at noon (near to Groundhog Day), stake the Groundhog in the ground and mark where the sun shines through the hole.

Groundhog Day Shadow Tracker placement and sun beam

Time goes by

Every week or so, the sun will climb higher in the sky and the groundhog’s shadow will get shorter. You can visit the shadow tracker weekly and add a new marker each time. You will see the shadow getting shorter and shorter.

Groundhog Day Shadow Tracker showing shadow getting shorter with time

A new star will appear in the sky in 2022

Binary star merge to form a new star - a star in a starAn amazing thing is about to happen! A Star in a Star will be born.

You  can witness the birth of a new star in the night sky.  The new Star already has a birthday: 2022!

According to scientists, the new star will form when two stars that are orbiting each other grow so close that they merge into one.

The new star will appear in the constellation Cygnus. You can see the approximate location marked by a red circle in the Stellarium screenshot here:

A screenshot of Stellarium showing the new star location under the constellation Cygnus's left wing

The image of the two blue stars on the home page of this Star in a Star site shows what scientists think it looks like now. This impressive video shows an artist’s rendering of the star merge.

In the pair, one star is larger than the other so in the end, there will be a new star…  A Star in a Star.

The Moon Dance – Learn Moon Phases


The moon dance helps you learn and understand the phases of the moonAt sunset.

Face the sun.

Point your right hand toward the Sun.

Now point your left hand toward the Sun.

Both hands should be pointing at the sun.

Now, turn a little bit to the left, keeping both hands pointing at the sun.

Swing your left arm out until it is pointing in the opposite direction from the sunset. Bring your hands together again and repeat this swinging motion.

Bring your hands together again and point them both at the sun. This time, instead of swinging, bounce your left hand, leftward, some number of times… (14 is perfect if you can do it) …until it is pointing in the completely opposite direction from the Sun at sunset. Your left hand should now be pointing East.

Was the moon at any one of those 14 hand bounce spots? That is the age of the moon in days.

Please post your questions in the comments!