Galaxy Horizon – 4/20 at Midnight

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. galaxy horizon visualization. Shows the galaxy plane lined up with the local horizon at midnight on April 20th. This happens every year.
The Milky Way galaxy lines up perfectly with your local horizon view on April 20th (4/20) at midnight each year.

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.

How can I see Venus?

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.

See Venus, see the plane of the solar system

Solar system plane
The planets all orbit the sun in the same plane. Image not to scale.

All the planets in the solar system orbit the sun. All of the orbits line up in neatly nested concentric rings. And all of the rings are lined up with each other as if they are placed on a single surface together.

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 see the orbit of Venus in the western sky just after sunset. See how the line from Venus to the Sun defines the ecliptic.
See Venus and you can see the orbit of 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.


Venus information from In the Sky.

How many stars are in the sky? A Quiz

Have you ever looked up and wondered how many stars are in the sky?

This person is wondering how many stars are visible in the sky right now.
How many stars are in the sky right now?

It’s a simple question with a surprising answer!

This question has layers of answers – the most common answer is “it depends” – it depends on where you are, what part of the year it is, what time it is, and most of all… how dark are your skies and how powerful is your telescope? 

Let’s start with a quick (approximate) list of how many stars you might be able to see at once at night under “normal” conditions with no telescope. (The details on these calculations are near the end of this article.)

How many stars can I see at night:

  • The most stars anyone can see from the Earth (no telescope): 3700
  • The number of stars can you see at a dark sky site: 2000
  • How many stars can I see in my suburban yard: 200
  • How many stars can I see in a large city: 20

Another trick answer to “how many stars are in the sky?” is: “all of them!” All of them are in the sky, but they are just not visible to you “right now” for one reason or another.

I know, these answers are not quite what you may be looking for. Let’s look a little bit deeper by starting our search for how many stars can I see at night.

Quiz – How many stars are in the sky, how well do you know them?

Let’s start by taking a short quiz (answers are just a scroll away, so don’t peek!):

  1. Which star is the brightest in the sky?
  2. Which star is the closest to us?
  3. Which star is the first one discovered to have a planet supporting life?
  4. Which stars have a solar system?

Scroll down a bit to see the answers to this first part of the quiz.

While we are waiting to scroll (and to keep the peekers from peeking!)… let’s talk a little bit about how professional astronomers count the stars.

Annie Jump Cannon – Harvard star counter extraordinaire

Astronomer Annie Jump Cannon found a lot of stars and classified them
Annie Jump Cannon – Harvard University Image: Public Domain

Have you ever tried to count the stars? If you have, you probably gave up at some point because there were too many to keep track of.

In the early 1900s there was a tenacious Harvard astronomer named Annie Jump Cannon who didn’t give up counting! In fact, she counted so many stars that she almost lost track.

To keep everything organized she started categorizing them by their colors. She invented the spectral classification system – O,B,A,F,G,K,M – and personally classified over 350,000 stars!

Her method of categorizing stars is still in use today.

The answers to the star Quiz

The Sun is one of many stars. Proxima Centauri, Sirius, Arturus and Rigel are shown in a size comparison.
The Sun compared to other stars. Image: Daniel Cummings

The number of stars in the sky is…

SPOILER ALERT! – Here are the answers to the quiz above! 

The quiz is a trick quiz because the answers to are all “the Sun.” If we don’t include the Sun in this quiz then we get very different answers and the answers are:

  1. Sirius is the brightest star (it is almost as bright as Jupiter),
  2. Proxima Centauri is the closest star (it is 4.22 light years away),
  3. No star’s exoplanets have yet been proven to support life – the Sun is the only star so far to host life.
  4. There is only one solar system. The answer to question 4 is truly a trick question: “No other stars have a solar system… because only one star is named “Sol” … the Sun! The “Solar System” is named after Sol – the Sun. See: Sol+ar = Solar. Other stars would have planetary systems named after the star. For instance, there may be a “Sirius-ar system” that hosts a planet like Earth.

The Known Universe

Ok, now you are done with the quiz let’s get counting stars. Go outside, look up, start counting!

How many stars are in this picture? We may never know since the know universe keeps growing
Artist’s conception of the known universe. Credit: Pablo Carlos Budassi via Wikipedia

Before, we start… There is one tiny problem with counting the number of stars. Even with the best tools humans have invented, we can only see a small part of the universe. Astronomers call this the “known universe” and it stretches about 14 billion years back through time in one direction, and (we assume) 14 billion years back in another direction.

Ten sextillion stars in the known universe

Astronomers have estimated the number of stars in the Milky Way galaxy as one hundred billion (this is a low estimate by the way). The Milky Way is an average galaxy. Astronomers estimate that there are one hundred billion galaxies in the known universe (another low estimate). The math to figure out the total number of stars in the universe is as simple as the result is mind-boggling!

100,000,000,000 (one hundred billion – 11 zeros) x 100,000,000,000 (one hundred billion – another 11 zeros) = 10,000,000,000,000,000,000,000 (ten sextillion – 22 zeros)

Counting to one million is not like Counting to one billion

A million is way, way smaller than a billion.

It would take 11 days to count to one million if you counted one number per second without doing anything else. Counting to a billion at the same pace (one per second) would take you 32 years of continuous counting!

Count to 1 million at a rate of 1 per second = 11 days
Count to 1 billion = 32 years

– Arithmetic

If you counted all the stars in the Milky Way at the rate of one per second, it would take you about 3200 years to finish counting our one galaxy’s set of stars.

How many stars in my sky now?

Ok. So, let’s get real. Now we know how many stars there may be in the entire known universe, let’s narrow the question. 

Here is how we will narrow the question. Let’s create a set of ideal conditions that will guarantee we see as many stars as possible.

This is the set of almost ideal seeing conditions. This set of conditions would guarantee seeing the most number of stars that anyone could ever see with the naked eye:

I am on a tall mountain top with no trees looking up at the perfectly clear and calm sky right now. It is midnight. I can now see approximately half the stars in the sky. These near perfect seeing conditions make it so that I can see stars to magnitude 6 with my 20/20 vision.

Number of stars in the sky I see

According to David Haworth of there are approximately 7400 magnitude 6 or brighter stars. Remember, we can only see half the sky at any time. So, cutting 7400 in half we get 3700 stars. 3700 stars is the greatest number of stars that any earth-bound human can ever see at any one moment without a telescope or other aides.

Stars in my sky right now

Now we know how many stars you might ever see at once, let’s look at how most people see the sky.

Most people cannot see 3700 stars because of serious light pollution, horizons full of trees and landforms, and atmosphere, lots of watery, moving atmosphere floating above us.

If you are lucky and go camping in a dark sky area you might be able to see about 2000 stars on a good night.

In a typical, light-polluted suburban sky you can see approximately 200-300 stars.

In the middle of a big city the number of stars goes down to about 12 to 20.

12 Stars.

12. That’s not a lot of stars. Maybe take a trip out of the city to see the stars?


You can see 3700 stars under perfect viewing conditions.

You can see about 2000 under excellent dark skies.

You can see about 300 stars in the suburbs.

You can see 12 stars under bright city lights.

References for how many stars – David Haworth compiled data on the number of stars at each magnitude and presented it in a nice table.

Sky and Telescope article – covers skyglow, light measuring technology, light maps, NELM and more. Authoritative. article – how astronomers estimate the number of stars in the Milky Way.

National Geographic Kids – Mini article on light pollution with estimates on how many stars are visible from different places.

8 Ways to Find the North Star

Find the North Star

You can use these 8 ways to find the North Star (Polaris). These are all fun ways to find the North Star.

Can you find the North Star in this image? It is the star that is closest to the middle of the concentric rings of star trails. This is a long exposure photograph of real stars as seen over the course of several hours during the night.
A long exposure photograph showing all of the other northern sky stars circling around the North Star. Image provided by ESO.

8 ways to find the North Star

  1. Look north and guess – you can find the North Star in a relatively dark region of the sky and there are not many other bright stars around it. If you are south of the equator, head north before you try to look for the North Star because you won’t be able to see it until you get the Earth out of the way.
  2. Use the Big Dipper cup stars as pointers. This is the classic way to find the North Star. The two stars of the Big Dipper cup are known as the “pointer stars” and they show you which star is the North Star. The North Star is about 5 lengths of the pointer stars away.
  3. Camera timelapse – ooh! I love timelapse. A great timelapse of the night sky is an unbeatable way to relax. By taking a timelapse of the starry sky you can detect the apparent motion of the stars. If your timelapse covers enough of the sky  (with a wide angle view) chances are that you will be able to identify the North Star because it is the star that moves the least.
  4. Phone app – grab a planetarium app like SkySafari. Almost every star app these days has a “Augmented Reality” view that you can use to find Polaris. Just use the AR method of holding the phoone up above your head and searching around or you can type the name of a star into the search box in the app.
  5. Observe the sky, patiently measuring the movement of every star. The one that moves the least is Polaris. This might take a long time because the stars move pretty slowly.
  6. Mark a known spot as your North Star viewing spot. This is easy to do with a product like the Star Spot. You can return to that spot any time of day or night  to sight the star – the North Star is always in the same place in the sky.
  7. The North Star is located in between the two easy-to-identify constellations The Big Dipper and Cassiopeia – the Queen.
  8. Memorize its color and the stars around it – this is easier than it sounds! Polaris is a yellow supergiant and has a faint yellow tint. Also, the North Star is located in a region of the Milky Way that has fewer stars so it is surrounded by dark areas of the night sky.

Here is the classic way to find the North Star! Use the pointer stars of the Big Dipper. This is a reliable method for finding the North Star that has been taught to generations.

Find the Big Dipper to find the North Star

Look at the two stars in the picture below. One is Dubhe – which is labeled a for alpha, and the other Merak – which is labeled b for beta. These form the outer lip of the Big Dipper’s cup. These two stars can be used to create an imaginary line to “point” at the North Star.

The distance from the pointer stars to the North Star is about 5 times the distance between Dubhe and Merak.

The North Star is shown in this image as a red dot labeled “Polaris.”

You can find the North Star using the two pointer stars of the Big Dipper (Dubhe and Merak) The North Star is also called Polaris and is part of the constellation Ursa Minor.
Use the two pointer stars of the Big Dipper to find the North Star (Polaris). Picture credit user Bonĉ source Wikipedia

These instructions work for the 80% of people who live in the northern hemisphere – anywhere north of the equator. For the 20% of people who live in the southern hemisphere the North Star is not visible because it is blocked by the Earth. As you move south toward the equator (and eventually move past the equator), the North Star gradually sinks lower in the sky until it stays completely below the northern horizon.

Click here to continue reading…

Where is the Sun?

Where is the Sun right now?

Can’t see the Sun? Maybe there is something blocking it. Here is a list of 10 surprising things that can block the Sun.

IMAGE of floating/flying things overhead (in roughly size order) that can block all or part of the SUN: Flying animals (Bugs, Birds/Flying Mammals), Flying objects (Drones/Balloons/Airplanes/Helicopters/Rockets/Bombs/Blimps), Smoke/Clouds, Spacecraft (Satellites/Space Stations/UFOs), Asteroids, Moon, Mercury, Venus, Earth.
Things that block the Sun.

Where is the Sun during the day? On a clear day, this is a very simple question. The Sun is “up there” in the sky – it’s a big, bright, fiery ball and it’s generally a yellowish orange color. You just point to it – there it is, up in the sky, the Sun.

However, many things can block the Sun. Usually, it is clouds that block the Sun, but not always. Let’s take a tour of the astonishing number of things that can block the Sun.

Click here to continue reading…

GPS uses Quasars

GPS uses Quasars to work

GPS uses quasars to get its own position
GPS satellite in low Earth orbit. Credit: NASA artist’s impression
The Global Positioning System (GPS) is precise.

That precision originates in a mind-blowing place!

GPS signals tell billions of people where they are each day.

GPS helps pilots land planes and captains steer ships.

GPS signals stamp locations onto millions of photos.

GPS gives you driving directions.

But, where does GPS get its own GPS?

The GPS for GPS

How does GPS know where a GPS satellite is?Click here to continue reading…

Astronomy Koan

Astronomy Koan – Definition

A family views the visible orbit path of the earth.
A family views the visible orbit path of the earth.

An astronomy koan is a short, easy-to-memorize phrase that distills a key teaching about astronomy (especially physical astronomy).

The words are simple enough for a child to learn, but they carry complex insights about scientific observations.

The astronomy koan is a mnemonic that has layers of meaning or presents an ambiguous or challenging observation in a pithy phrase.

Try memorizing one of these – you can bring these with you everywhere.

Four Physical Astronomy Koans

That star rose earlier today.

The moon moves toward the dawn.

Night is where you are.

Same Sun all night. Dusk to the left, dawn to the right.

What do you think about these?

Do you know any astronomy koans?

Share your ideas in the comments section.

Stellarium – Find the Humanity Star

A new satellite called the Humanity Star looks like a disco ball

Update Apr. 1, 2018.

The Humanity Star is no longer an active satellite – it has fallen out of the sky:

However, even though the Humanity Star is now gone, this article still teaches you how to load satellites into Stellarium. So, read on for a quick tutorial on how to track any satellite in Stellarium.

Looking for information on how to track the Humanity Star any satellite location using Stellarium astronomy software?

Here is a quick tutorial on how to find the Humanity Star any satellite using Stellarium.

You can track the position of the Humanity Star with Stellarium
The Humanity Star satellite before launch – still on the Earth.

Rocket Lab founder and CEO Peter Beck announced that the company’s rocket had placed a special satellite in a 90 minute orbit around the Earth.

A lot of people want to see this new “star” in the sky. Stellarium can help you do that.

If you don’t have Stellarium, you can download a copy here. And I have a few tutorials that can help you get started with this amazing piece of software.Click here to continue reading…

Quiz – Can we see the Sun at midnight? Where is the Sun?

Can we see the Sun at midnight?

Yes. We can see the Sun at midnight. But, only if we are at one of the polar regions during the Summer season.

A quiz – seeing the Sun at midnight – don’t scroll til you try to answer!

When you see the Sun “rising” in the morning at dawn you are facing the east.

When you see the Sun “setting” at the end of the day you are facing the west.

Assuming you are not above the Arctic circle and not too close to the equator… if you could look right at the Sun (when the night is exactly half over) – by looking through the Earth – which cardinal direction would you be facing? East, West, North, or South?

Looking north or south to see the Sun at midnight?
A compass rose showing the cardinal directions.

Click here to continue reading…

See Mercury and Venus orbits during the day

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.

Use your hands and elbows to see Mercury and Venus orbits any time of day or night. The orbits of Mercury and Venus can be seen.
Two hand spans show the orbit of Mercury, elbows show the orbit of Venus.

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.

Click here to continue reading…