The Size of the Universe
Friday, 12 May 2017 | Neil Parker
The Solar System
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.
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.
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
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 Gas Giants
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
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
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
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 Galactic Neighbourhood - 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.
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."