The Planets

Pttm, Going Postal

Part Three- Earth

In this third part of a series of nine articles, today  we look at the planet Earth.

The Prologue:

It’s round(ish), not flat.

Pttm, Going Postal

The Earth and the far side of the moon on July 5, 2016, also featuring Typhoon Nepartak over the Pacific Ocean, imaged by NASA’s Deep Space Climate Observatory (DSCOVR) satellite, about 1.5 million km (930,000 mi) from Earth,  One of my personal favourite pictures, told you I like “specialist sites”

Throughout this series, I would like to give a huge H/T to NASA.  I have used other sources as well, such as  and  which is a great site for showing you where to look.


Earth is the third planet from the Sun and is the largest of the terrestrial planets. The Earth is the only planet in our solar system not to be named after a Greek or Roman deity. The Earth was formed approximately 4.54 billion years ago and is the only known planet to support life.  Watch this space though.

Equatorial Diameter: 12,756 km
Polar Diameter: 12,714 km
Mass: 5.97 x 10^24 kg
Moons: 1 (The Moon)
Orbit Distance: 149,598,262 km (1 AU)
Orbit Period: 365.24 days
Surface Temperature: -88 to 58°C

-70% of the Earth’s surface is covered in water.  Earth is mostly iron, oxygen and silicon; If you could separate the Earth out into piles of material, you’d get 32.1 % iron, 30.1% oxygen, 15.1% silicon, and 13.9% magnesium.  Of course, most of this iron is actually down at the core of the Earth.  If you could actually get down and sample the core, it would be 88% iron. 47% of the Earth’s crust consists of oxygen.

Speaking of water, only 3% water of the earth is fresh, the rest, 97% is salted. Of that 3%, over 2% is frozen in ice sheets and glaciers which means less than 1% fresh water is found in lakes, rivers and underground.

Earth doesn’t take 24 hours to rotate on its axis.  It’s 23 hours, 56 minutes and 4 seconds. This is the amount of time it takes for the Earth to completely rotate around its axis; astronomers call this a sidereal day.   Also a year on Earth isn’t 365 days
It’s 365.2564 days. It’s this extra .2564 days that creates the need for leap years. That’s why we tack on an extra day in February every year divisible by 4 – 2004, 2008, etc – unless it’s divisible by 100 (1900, 2100, etc)… unless it’s divisible by 400 (1600, 2000, etc).

I mentioned in an earlier article about being able to see other planets, sometimes we can, sometimes not.  This is because our Planet, like all the others, are in orbit, rotating around the sun, and all spin as well.

The sun also has an orbit of its own in the Milky Way.  The sun is about 25,000 light-years from the center of the galaxy, and the Milky Way is thought to be about 100,000 light-years across.  We are thought to be about halfway out from the centre.  The sun and the solar system appear to be moving at 200 kilometers per second, or at an average speed of 515,000 mph (828,000 km/h). Even at this rapid speed, the solar system would take about 230 million years to travel all the way around the Milky Way.

The Milky Way, too, moves in space relative to other galaxies. In about 4 billion years, the Milky Way will collide with its nearest neighbor, the Andromeda Galaxy. The two are rushing toward each other at about 70 miles per second (112 km per second).

Anyway, getting back to Earth:

Earth’s spin is constant (well nearly, it is in fact gradually slowing) but the speed depends on what latitude you are located at. Here’s an example. The circumference (distance around the largest part of the Earth) is roughly 24,898 miles (40,070 kilometers), according to NASA. (This area is also called the equator.) If you estimate that a day is 24 hours long, you divide the circumference by the length of the day.  This produces a speed at the equator of about 1,037 mph (1,670 km/h).

Just in case you are worrying abut the spln slowing, there will eventually be 25 hours in one day, but not for another 140 million years, the rate of deceleration is approximately 17 milliseconds per hundred years.

In an earlier article on the Planet Mercury I mentioned the tilt of the planet was about ​130 degree.  Our planetary axis today is tilted at  23.5 degrees from the plane of its orbit around the sun. But this tilt changes. During a cycle that averages about 40,000 years, the tilt of the axis varies between 22.1 and 24.5 degree.  It is this tilt that gives our seasons:

Pttm, Going Postal

Earth has a powerful magnetic field.  This phenomenon is caused by the nickel-iron core of the planet, coupled with its rapid rotation.  This field protects the Earth from the effects of solar wind.

We truly are in the Goldilocks zone

Where to find it:

Well, you are on it.  To give a perspective though, as of the 1st January 2018 we are here:

You can follow NASA’s stream directly here: If the screen is black, don’t worry — the space station is likely just on Earth’s night side. (The station completes one orbit every 90 minutes, so you won’t have to wait too long for our gorgeous planet to roll into view once again.)

© Phil the test manager 2018