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The Size of the Universe

Friday, 12 May 2017  |  Neil Parker

The Solar System
The Sun and Moon

Everyone has trouble understanding the scale of the solar system and the wider universe. Distances are mind-bogglingly difficult to comprehend. The profusion of different units such as kilometres, astronomical units, light-years and parsecs add to the confusion, although they help astronomers reduce the number of zeroes at the end of the numbers they deal with.

One of the things I have found useful is to use ratios of sizes and distances to help get things into perspective. I usually work to the nearest round number as I'm not very good at remembering facts and figures. For example, I carry around in my head the fact that the diameter of the Sun is about 100 times the diameter of the Earth (it's closer to 109 times) and the distance from the Earth to the Sun is about 100 times the diameter of the Sun (it's closer to 108). I can picture 100 Earth's set side by side across the Sun's equator and when I'm looking at the Sun through a telescope (fitted with the appropriate filter) I can estimate how big a flare, a sunspot or any other feature is compared to the size of the Earth.

100 Suns

I try to picture 100 Suns side by side filling the gap between the Sun and the Earth. This also helps me get a feeling for its size. The line above shows 100 Suns side by side. On this scale the Earth would be a small fraction of one pixel.

The Earth is about four times the diameter of the Moon, which makes the Sun about 400 times the diameter of the Moon. Coincidentally, the distance from the Earth to the Sun is about 400 times the distance from the Earth to the Moon, which is why the Moon just covers the Sun during a total solar eclipse.

Diamond Ring
Just as the Moon covers the Sun at the start of totality in a Solar Eclipse, Sunlight streaming through a gap in the Moon's mountains causes the Diamond Ring Effect. You don't need sophisticated equipment to photograph an eclipse. The photo on the left was taken by John Ruddick in Turkey with this simple setup

Mercury and Venus
We've been lucky in recent years having a transit of Mercury and a transit of Venus across the face of the Sun. The transit of Venus was the first one seen by anyone living today. Transits give us a unique opportunity to see the relative size of the two inner planets directly. However, we still need to think carefully about relative distances if we are not to be deceived.

When Mercury passes between us and the Sun its distance from the Sun is just under two-fifths of the Earth's distance from the Sun. This makes it appear bigger than it really is (about 1.7 times bigger). Venus is almost three-quarters of the way from the Sun to the Earth during a transit so it appears nearly four times bigger than it really is, compared with the Sun. I'd forgotten this when I first saw the transit and the apparent size of Venus surprised me.

The position of Venus in its orbit affects how big it appears to us. (The same is true of Mercury but the effect is less noticeable and Mercury is harder to observe, being so close to the Sun.) At its closest approach to Earth, Venus is about a quarter of the distance to the Sun. At its furthest point, i.e. at the other side of the Sun, it is about one and three quarter times the distance to the Sun. Its size thus varies by about seven times. We can't see all this variation as we can't see Venus when it is close to the Sun, either in front or behind it, but we can see a substantial part of the variation. You might like to follow Venus through a complete cycle to see this for yourself.

Mercury and Venus Transits
Rick Beno took the time-lapse shot on the left from the Arizona Sky Village. The line across the lower part of the Sun is Mercury's silhouette moving. The large black area on the left is a sunspot. On the right, Venus looks about four times bigger than it would if it were the same distance as the Sun

Mars
Like Venus, Mars' apparent size varies substantially depending on where it is in its orbit. Unlike Venus, we can never see it transit across the face of the Sun as it is always further from the Sun than we are. This is true of all the other planets; Mercury and Venus are the only ones closer to the Sun than the Earth is.

Mars' orbit is about half as big again as the Earth's, so its years are longer and we keep overtaking it on the inside, about every 26 months. As we pass by the distance to Mars is about half the distance to the Sun. When we are on the opposite side of the Sun then the distance to Mars is about two and a half times the distance to the Sun, making the variation in apparent size about five times between one extreme and the other. This is why planetary observers and photographers get excited when Mars is at 'opposition', i.e. the Earth is directly between it and the Sun and so at its closest to Mars.

The Asteroids
Between the orbits of Mars and Jupiter we find the asteroid belt, a collection of rocky fragments orbiting the Sun where we might expect to find another planet. One theory is that this collection of debris failed to coalesce into a planet due to the disturbing influence of Jupiter's gravitational field. Most of the debris is in orbits about 2.7 times the Earth's orbit. It is estimated that the mass of the Earth is about 1,000 times greater than all the asteroids put together.

The Gas Giants
Jupiter, Saturn, Uranus and Neptune are the four giant planets composed almost entirely of gas. Only Jupiter and Saturn were known until modern times as Uranus and Neptune are such a long way from the Sun that optical aids are needed to see them. Their orbits respectively are 5.2, 9.5, 19.2 and 30 times the diameter of the Earth's orbit. This makes their years much longer as well. In round figures their orbits respectively are 12, 30, 84 and 164 Earth years.

Jupiter's year of 12 Earth years means it passes through one sign of the zodiac each Earth year. We thus see it against a different part of the sky each year and our closest approach to it moves through the seasons. This means it is better placed for observing in some years than others. Apart from this, its appearance varies little from year to year.

Saturn, on the other hand does change in appearance because we see its rings from different angles from year to year. If we see them from beneath in year one then about seven and a half years later they will be edge on. Seven and a half years later we will see them from above, then edge on, then from below and so on. One complete cycle takes around 30 years so a typical person observing Saturn throughout their adult life would see two complete cycles.

Most people would not be able to observe Uranus travel through a complete orbit as it takes 84 years and no-one can observe Neptune's complete 184-year orbit.

Pluto and the other Dwarf Planets
Many of us grew up believing that Pluto was the ninth planet but discoveries of many similar-sized bodies, plus Pluto's marked differences from the other outer planets, have caused astronomers to reclassify it. Its highly eccentric orbit varies between 30 and 49 times the Earth's orbit. At its closest it is nearer the Sun than Neptune.

Pluto is now considered to be part of the Kuiper Belt, a collection of rocky objects orbiting between 30 and 55 times the Earth's orbit. It is similar to the asteroid belt but is about 20 times as wide and 20 to 200 times as massive (estimates of the mass vary widely).

The Oort Cloud
Way beyond the Kuiper Belt it is believed there is a shell of comet-like objects called the Oort Cloud. Long-period comets such as Halley are believed to have originated there and it provides a never-ending source of new comets to replace the ones destroyed by repeated passes close to the Sun. I had the privilege of meeting Jan Oort on one of my visits to Dwingeloo; he revived the idea of the cloud in 1950, the year of my birth.

There is no direct observational evidence for the Oort Cloud but it is believed to be about 1,000 times further away than Pluto, almost one light year from the Sun. This puts it a quarter of the way to the nearest star and marks the outer edge of the Solar System.

NASA's space craft 'New Horizons' has now passed Pluto and is heading for the Kuiper Belt. It reached Pluto in 2015, 14 years after launch. It started its journey at 36,000 miles per hour and accelerated after passing Jupiter. If it continued travelling after passing the Kuiper Belt it would take around 14,000 years to reach the Oort Cloud.

Ernst Opik, Jan Oort and Lembit Opik
Ernst Opik proposed a theory in 1932 that comets originated in a cloud way beyond the orbit of Pluto. Originally from Estonia he fled the Red Army and took a post at Armagh in Northern Ireland. Jan Oort revived Opik's idea in 1950 and the cloud usually bears his name, although it's sometimes referred to as the Opik-Oort Cloud. Lembit Opik is the grandson of Ernst Opik and as a Liberal Democrat MP was an advocate for the Campaign for Dark Skies but is perhaps better remembered for his interest in 'The Cheeky Girls'.

Beyond the Solar System
Nearby Stars
The nearest Star to our Sun is Proxima Centauri. You may remember I said that a line of 100 Suns would fit between the Sun and the Earth. It would take around 28.5 million Suns lined up to bridge the distance to Proxima Centauri. Light takes around 4.2 years to reach us from Proxima Centauri and the New Horizons Space Craft would take around 60,000 years to get there.

Of the 25 brightest stars in the night sky, Deneb in Cygnus the Swan is the furthest away. Estimates of its distance vary enormously but it could be as close as 500 times the distance to Proxima Centauri or as far as 1,800 times the distance.

The Milky Way Galaxy
Our Sun is just one of an estimated 200 to 400 billion stars that make up the Milky Way Galaxy. Its diameter is estimated to be somewhere between 17,000 and 25,000 times the distance to the nearest star. The Milky Way Galaxy is disc-shaped with a bulge at the centre around which the stars, including our Sun, rotate. Our Sun's distance from the centre is estimated to be between one half and two thirds of the way towards the edge of the Galaxy.

Our Galactic Neighbourhood - The Local Cluster
Our Galaxy is one of a small group called the Local Cluster. It includes the Magellanic Clouds seen from Earth's southern hemisphere, and Andromeda, the most distant object we can see with our naked eye. Whilst the distances between stars in a galaxy are enormous compared to the sizes of the stars themselves (remember the distance from the Sun to the next nearest star is 28.5 million times the diameter of the Sun) distances between galaxies are relatively small compared to their sizes. If we made models of galaxies the size of dinner plates then we would need to place them just a few feet apart to produce a true scale model of the local cluster.

Our Galaxy and the Andromeda Galaxy are falling towards each other and will 'collide' in the dim and distant future. But, because of the enormous distances between their individual stars, few if any direct star collisions will occur. The two galaxies will pass through each other but gravitational forces will alter their shapes considerably. Examples of galaxies that have already collided can be seen in the night sky.

The Universe
When astronomers talk about the size of the Universe they often qualify it by including the word 'visible' to indicate that there may be more beyond what we can see. At present we believe the Universe to be about 13.7 billion years old. Based purely on how far light would travel in that time produces one estimate for the size of the Universe, i.e. around 27 billion light-years or 270 million times the diameter of the Milky Way Galaxy.

This simplistic approach doesn't take the expansion of the Universe into account and some other estimates say the Universe is at least 156 billion light-years across, making it 1,560 million times bigger than the Milky Way Galaxy. As that famous son of Cambridge, Douglas Adams said, "Space is big. You just won't believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space."

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