November 26th, 2015

Why doesn’t the Moon smash into the Earth?

Because it’s in orbit. But can you explain why this is?

I’m currently writing two non-fiction children’s books about the Universe and Earth. In order to explain certain concepts simply, I’m finding I have to gain a much deeper understanding of them myself. Proof of what Einstein said: ‘If you can’t explain it simply, you don’t understand it well’.

Searching for explanations to the question of why the Moon doesn’t get pulled into the Earth by its greater gravity, I found a lot of perplexingly vague answers. But I also found this excellent explanation by one Mark Eichenlaub on StackExchange, illustrated with a beautiful diagram by Isaac Newton:

The moon does not fall to Earth because it is in an orbit.

One of the most difficult things to learn about physics is the concept of force. Just because there is a force on something does not mean it will be moving in the direction of the force. Instead, the force influences the motion to be a bit more in the direction of the force than it was before.

For example, if you roll a bowling ball straight down a lane, then run up beside it and kick it towards the gutter, you apply a force towards the gutter, but the ball doesn’t go straight into the gutter. Instead it keeps going down the lane, but picks up a little bit of diagonal motion as well.

Imagine you’re standing at the edge of a cliff 100m tall. If you drop a rock off, it will fall straight down because it had no velocity to begin with, so the only velocity it picks up is downward from the downward force.

If you throw the rock out horizontally, it will still fall, but it will keep moving out horizontally as it does so, and falls at an angle. (The angle isn’t constant – the shape is a curve called a parabola, but that’s relatively unimportant here.) The the force is straight down, but that force doesn’t stop the rock from moving horizontally.

If you throw the rock harder, it goes further, and falls at a shallower angle. The force on it from gravity is the same, but the original velocity was much bigger and so the deflection is less.

Now imagine throwing the rock so hard it travels one kilometer horizontally before it hits the ground. If you do that, something slightly new happens. The rock still falls, but it has to fall more than just 100m before it hits the ground. The reason is that the Earth is curved, and so as the rock traveled out that kilometer, the Earth was actually curving away underneath of it. In one kilometer, it turns out the Earth curves away by about 10 centimeters – a small difference, but a real one.

As you throw the rock even harder than that, the curving away of the Earth underneath becomes more significant. If you could throw the rock 10 kilometers, the Earth would now curve away by 10 meters, and for a 100 km throw the Earth curves away by an entire kilometer. Now the stone has to fall a very long way down compared to the 100m cliff it was dropped from.

Check out the following drawing. It was made by Isaac Newton, the first person to understand orbits. IMHO it is one of the greatest diagrams ever made.


What it shows is that if you could throw the rock hard enough, the Earth would curve away from underneath the rock so much that the rock actually never gets any closer to the ground. It goes all the way around in the circle and might hit you in the back of the head!

This is an orbit. It’s what satellites and the moon are doing. We can’t actually do it here close to the surface of the Earth due to wind resistance, but on the surface of the moon, where there’s no atmosphere, you could indeed have a very low orbit.

This is the mechanism by which things “stay up” in space.

Gravity gets weaker as you go further out. The Earth’s gravity is much weaker at the moon than at a low-earth orbit satellite. Because gravity is so much weaker at the moon, the moon orbits much more slowly than the International Space Station, for example. The moon takes one month to go around. The ISS takes a few hours. An interesting consequence is that if you go out just the right amount in between, about six Earth radii, you reach a point where gravity is weakened enough that an orbit around the Earth takes 24 hours. There, you could have a “geosynchronous orbit”, a satellite that orbits so that it stays above the same spot on Earth’s equator as Earth spins.

Although gravity gets weaker as you go further out, there is no cut-off distance. In theory, gravity extends forever. However, if you went towards the sun, eventually the sun’s gravity would be stronger than the Earth’s, and then you wouldn’t fall back to Earth any more, even lacking the speed to orbit. That would happen if you went about .1% of the distance to the sun, or about 250,000 km, or 40 Earth radii. (This is actually less than the distance to the moon, but the moon doesn’t fall into the Sun because it’s orbiting the sun, just like the Earth itself is.)

So the moon “falls” toward Earth due to gravity, but doesn’t get any closer to Earth because its motion is an orbit, and the dynamics of the orbit are determined by the strength of gravity at that distance and by Newton’s laws of motion.

Still, it will be hard to condense this into a few short sentences for kids.

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May 31st, 2014

look closer…


Amazing GIF image created using images from a scanning electron microscope:

As the incredibly powerful microscope zooms in, it goes from showing an amphipod (a type of shell-less crustacean), to a diatom (a type of algae) that’s on the amphipod, to a microscopic bacterium that’s on the diatom that’s on the amphipod. It’s life, on life, on life:

The GIF was created by James Tyrwhitt-Drake back in 2012, when he captured the images at the University of Victoria’s Advanced Microscopy Facility and posted the final product to his Tumblog, Infinity Imagined.

Via PetaPixel.

May 8th, 2014

Psychogeography and the dérive


…The situationists’ desire to become psychogeographers, with an understanding of the ‘precise laws and specific effects of the geographical environment, consciously organized or not, on the emotions and behaviour of individuals’, was intended to cultivate an awareness of the ways in which everyday life is presently conditioned and controlled, the ways in which this manipulation can be exposed and subverted, and the possibilities for chosen forms of constructed situations in the post-spectacular world. Only an awareness of the influences of the existing environment can encourage the critique of the present conditions of daily life, and yet it is precisely this concern with the environment which we live which is ignored.


Psychogeography on wikipedia

November 30th, 2013

Michio Kaku

Kaku tells a good story.

November 7th, 2013

I know I don’t need to believe, I know

August 24th, 2013

Apollo computer

A fascinating insight into how astronauts would have interacted with the Apollo’s computer. (via kottke)

January 2nd, 2013

NASA cribs

(via kottke)

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August 25th, 2012

the ‘feel’ of being in Paris or London

Mind-boggling image research allows us to create time and site specific portraits of city architecture and make comparisons. Very impressive! via ScienceDump

May 29th, 2012

astronaut photoblog

Urenlang Dragon in de gaten houden tijdens de nadering is geen straf. Hier over Namibie.

Hours on end monitoring Dragon’s approach is no punishment. Here over Namibia.

The FlickR photostream of Dutch astronaut Andre Kuipers is a unique and visually beautiful insight into life in space…

See here.

April 2nd, 2012

sound in water

Started thinking about making an underwater sound installation and found that this video serves as a nice introduction to the understanding of sound’s behaviour in water.

November 17th, 2011


The International Space Station has a nice camera on board these days… View fullscreen.

Time Lapse View from Space, Fly Over | NASA, ISS from Michael König.

Another (see previous post) impressive time-lapse video found via kottke.

September 14th, 2011

saturn abides

If you look to the top left of Saturn, seen here eclipsing our sun, you can see the planet Earth as a tiny white dot in the background. The perfect order of such a massive object and the debris bound by it’s pull, and the perfection in turn of its alignment in space, is glorious.

From Nasa’s image of the day September 04 2011.

June 12th, 2011

ISS lamp

Wonderful idea!

June 8th, 2011

an explosion a million kilometres wide

As you can see in these videos, not only is some of the surface ejected into space as a result of the explosion, some of it returns to crash back into the Sun. The videos are being provided through which is an open-source project, funded by ESA and NASA, for the visualization of solar and heliospheric data. It seems the video of the solar flare was so popular on Tuesday that some visitors to had long movie waits due to the increase in traffic.

More video, photo and background info at

May 31st, 2011

the simian hunch

The European Space Agency has some insights into the unglamorous practical details of daily life as an astronaut. Snip:

Once stirred, the astronauts tend to adopt a foetus-like posture as they move weightlessly about the station. Sometimes referred to unflatteringly as the “simian hunch”, it seems to be the natural human attitude in microgravity; perhaps it really is an echo of the weightless months that every growing embryo spends floating in its mother’s womb.

The crew dress as quickly as they can: no easy task when your limbs float out at odd angles. They wear disposable clothes, replacing them once every three days: there are no washing machines in space. But the ISS does have a shower. Water squirts out of the “top” to be sucked down by an air fan at the “bottom”. The shower has to be used sparingly to conserve water, but it is a luxury item that earlier space pioneers would have envied. and today’s astronauts cherish.

More on the ESA website (via reddit)

April 21st, 2011

lacanian gaze

Wiki: Hieronymus Bosch’s The Conjurer. While other figures observe objects within the painting, the woman in green observes the viewer. The painting thus makes the viewer aware of being on display.

Gaze is a psychoanalytical term brought into popular usage by Jacques Lacan to describe the anxious state that comes with the awareness that one can be viewed. The psychological effect, Lacan argues, is that the subject loses some sense of autonomy upon realizing that he or she is a visible object. This concept is bound with his theory of the mirror stage, in which a child encountering a mirror realizes that he or she has an external appearance. Lacan suggests that this gaze effect can similarly be produced by any conceivable object such as a chair or a television screen. This is not to say that the object behaves optically as a mirror; instead it means that the awareness of any object can induce an awareness of also being an object.


Related: Men dream of women

February 4th, 2011

down here on earth

The Galaxy Song from Monty Python’s The Meaning of Life.

January 17th, 2011

vulnerable planet

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