Archives For Earth

Line 1. Let’s start with ‘typical’ humans. The average human adult male is 1.75 metres tall – that’s 3.83 cubits or 5.74 feet. The average female is 1.62 metres – that’s 5.4 light-nanoseconds or 0.008 furlongs.

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You live on Earth (Sol d, perhaps?). This is an Earth-like planet in a Sun-like star system. The third planet of eight in a rich system, including a least one planet populated entirely by robots (Mars, perhaps?). Earth is 12,742 km in diameter and thus has a circumference of 40,000 km or roughly 25,000 miles. Humans live in a thin layer (~20km) around the surface called the troposphere. If the Earth was a beach ball then all life on Earth exists within just 1mm around the surface.

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Through many years of international effort we have managed to keep a ‘space’ station in orbit – just above this troposphere – 1cm above the beach ball. But not high enough up that it can totally avoid the atmosphere – the ISS has to constantly boost itself back up because of air drag. We have sent just 24 people out into deep space, beyond the Earth’s atmosphere. All of then visited the Moon and the last ones returned in 1972: 42 years ago. They were all men, all white, and all American. We could do it again, we could do it better – but we chose not do so. (Mostly for political reasons IMHO.)

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Those astronauts visited the nearest body in space: the Moon – the second brightest thing in the sky . They were kind enough to return some photos to show us how teeny tiny we are, and how delicate out world really is. The Moon sits about a quarter of a million miles away (384,000 km). You could fit all the Solar System’s other planets in that gap.

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But that doesn’t include the Sun – the brightest thing in the sky. The Sun is truly huge. You can fit the Earth inside the Sun a million times. It has more than enough room for all the planets and then some. The Sun itself sits 93 million miles away – which means that light takes 8 minutes to reach us from the Sun. The Sun could have gone out 7.9 minutes ago and you’d only find out… now. Nope: we’re ok. For now.

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And yet we have flung robots into space and downloaded the images they have recorded. Sometimes we take extremely long-range selfies of a sort. Images of the Earth, of humanity reduced to a pixel or two. Here’s one from Mars, one from Saturn and one from out near the edge of the Solar System – taken by Voyager. These images collectively earn us the moniker ‘pale blue dot’. Out by Pluto, the Sun itself is has dimmed to look like an other stars. From Saturn, we are just a couple of pixels as seen by the Cassini probe:

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And truthfully, the Sun isn’t so special. In fact there are stars which make the Sun look even smaller than the Earth does here. VY Canis Major is staggeringly big – and could encompass the Sun 1,000,000,000 times. That’s a million trillion Earths. Oh and VY Canis Major isn’t even visible to the naked eye because it’s so far away that we can’t detect its photons without aid of telescopes or binoculars.

Which brings us to the Galaxy. The Sun is just one of hundred of billions of stars orbiting around the Milky Way. If the Sun was a blood cell then the Milky Way is the size of Europe. The Milky Way is staggeringly big also staggering diffuse – so much so that if you took two Milky Ways, and hit one with the other, then in all likelihood no two stars would collide. They would pass though each other like smoke.

In fact this will happen. The Andromeda galaxy – which is a lot like the Milky Way – is on a collision course with us. In about 4 billion years it will begin to merge with our galaxy in a spectacular collision. We see these happening elsewhere but the sheer scale of this vision in our own night sky makes me want to get a time machine and jump forward to see it happen. The Earth is unlikely to be affected by this, because of the lack of collisions – however our night sky will be spectacularly altered for hundred of billions of years. Makes you realise how dull it is right now. Just kidding!


But the Milky Way and Andromeda are just two out of hundred of billions of galaxies in the Universe. Gigantic stellar continents floating in a vast, void of almost nothing. Galaxies themselves form structures, and as we have looked deep into the cosmos we have seen one such structure: the Sloan Great Wall. A thick chain of galaxies, loosely bound to each other by gravity, stretching 1.4 billion light years across the Universe and about 1 billion light years from the Milky Way. It’s 1/60 of the Universe across. And yet there are even bigger thing out there.

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The largest known structure in the Universe is the Hercules–Corona Borealis Great Wall. At 10 billion light years across, this huge filament of galaxies in 1/10 the size of the observable Universe. It’s 100,000 time the size of the Milky Way, and 70 million trillion times bigger than than the Sun. We don’t have a good picture of it, but we know it’s there. It’s 7,000,000,000,000,000,000,000 times bigger than the Earth, which is very much bigger than you. I refer you to line 1.

This video is a time-lapse of images taken from a geostationary satellite. It shows a whole year of the Earth’s orbit around the Sun from 2010 to 2011. You can see the difference in illumination between the seasons created by the Earth’s tilt; the angle of the line between light and dark changes as we go around the Sun. This effect is caused by our axis of daily rotation being tilted by 23.5 degrees.

We are usually taught an approximate version of the truth when we learn about the Earth’s orbit. Most people are told that the Earth orbits the Sun like this:

Not Earth's Orbit

…however the orbit is not really circular, but slightly elliptical. It looks more like this:

Better Earth's Orbit

Not only is it slightly oval in shape, but it is also slightly off-centre. Our closest approach to the Sun each year is Perihelion (on the right of this image) and our farthest approach is Aphelion (on the left). Crucially though, the Earth is tilted in its own daily rotation by 23.5 degrees. This means that the North and South poles don’t line-up with the top-down view of thus image. So, sideways-on it looks like this:

Earth's Orbit Sideways

At around Perihelion (our closest approach to the Sun) the North Pole is pointing away from the Sun – actually it happens a couple of weeks before Perihelion. It is this effect that gives us our seasonal changes in temperature. When the North pole is pointed away from the Sun, as shown here, it means the Northern half of the Earth is receiving less energy from the Sun and so cools down and experiences more darkness. Six months later the Earth has moved around and now the North pole points more toward the Sun and thus it is the Southern Hemisphere that is darker and cools down.

Seasons in Earth's Orbit

It is not our distance from the Sun that determines the seasons, but our changing exposure to the Sun’s heat and light caused by our axial tilt. Our seasons are the result of the misalignment of our daily, North-South rotation compared with our yearly, Solar rotation. The darkest day is the Winter Solstice in the Northern Hemisphere – usually around December 21st – at the same time the Southern Hemisphere has its longest day. Perihelion occurs in early January (it was Jan 4th in 2014) which means we are closest to the Sun when it is coldest in the Northern Hemisphere and we are farthest from the Sun when it is hottest in the Southern Hemisphere.

The change in our tilt drastically changes how much of the Sun’s energy we receive, as is shown in the following photos of the Earth from Space. You can see that in Europe, for example, our share of daylight changes a great deal over the year. For the same reason, if you go far enough North or South there are places where it is continuously day or night for weeks or months at a time.

The above, amazing images of the seasons come from the same source as the video at the top of this post. They were taken by a EUMETSAT Earth observation satellite. This is a geostationary satellite, meaning that it looks at the same part of the world all the time. It stares at Africa and here you can see the sequence of Winter, Spring, Summer and Autumn in images taken at different times of the year:

The View from Saturn

November 18, 2013 — Leave a comment

Saturn Wave 1

This image was taken by Cassini, the amazing spacecraft that has been orbiting Saturn and its Moons for a decade. This image shows a view toward the Sun from Saturn – the most distant planet normally visible with the naked eye. As well as showing Saturn’s rings in all their glory, several of Saturn’s moons are visible in this shot. Perhaps more amazingly though, the Earth and Moon are seen as a bright spot in the lower-right and Mars and Venus can be seen in the top-left. On July 19th 2013 the world was asked to wave at Saturn as this image was taken (and many people did).

Saturn Wave 2

If you want to feel humanity’s astronomical significance to its fullest just think of this photo, and then think of what it took to be able to obtain it. An amazing achievement for the Cassini team.

[Image Credit: NASA/JPL-Caltech/SSI]


NASA have released the raw version of Friday’s #WaveAtSaturn photograph of the Earth from Cassini. This image shows the Earth and the Moon as two bright stars; it was received on Earth yesterday, July 20th.

The camera that took this image was 898,410,414 miles (1,445,851,410 km) away, on the Cassini probe orbiting the Saturnian system. A better, calibrated photo will be released in 2014.

[Image Credit: NASA/JPL/Space Science Institute]