For the past couple of weeks, I have been writing about the scale of things. From the very smallest to the very largest, all told, the Universe encompasses some 41 orders of magnitude.
But distance is only one way to measure the Universe. Equally important is time. Indeed, every night, as you and I look up at the stars, we are also looking back in time.
In some cases, back thousands or even millions of years.
Halfway between Cassiopeia and Pegasus, in the constellation Andromeda, there is a fuzzy patch that looks like a blurry star. It was originally designated "M31" but we now call it the Andromeda galaxy.
On a good night, it is a naked eye object and yet it is 2.5 million light years away.
To put that in perspective, the photons of light that you see when you look at Andromeda left there some 2.5 million years ago and have been travelling through space for all of that time only to land on a receptor in your eyeball and cause it to send a signal to your brain telling you that 2.5 million years ago, Andromeda was there. Humbling, isn't it?
Of course, the reality is that many photons from Andromeda are hitting us all the time. Indeed, every moment of every day, we are bombarded by light from even more distant objects. Billions of galaxies scattered across billions of light years of space each shine their light on us all the time.
This is not obvious because during the day, we can't see the stars, nebula, galaxies, and other celestial objects. The Sun - the closest star to the planet Earth - cast so much light upon us that it effectively turns our sky blue and blocks our view of the rest of space.
Still, the stars are there and their light shines on.
How do we know how far away galaxies are? Henrietta Leavitt developed a love for astronomy in school and she had a mind for numbers. She began working at the Harvard College Observatory in 1895 as a member of a group of women called the "Harvard Computers".
At the time, astronomers knew that stars come in all different sizes and colours. Their intensity also varied which made judging the distance to the stars difficult. Was a dim star small and close or intensely bright but really far away? Was a really bright star very close or a large star further away?
Leavitt became fascinated by one particular type of star called "Cepheid Variables". Mapping over 1700 stars in this class, she was able to show that the intensity of their light changed in regular patterns over periods that stretched from days to months.
More importantly, she discovered that there is a constant relationship between the length of a star's cycle and its absolute brightness. While this might seem a little esoteric, it gave astronomers a standard candle by which they could measure celestial distances. It provided astronomers with a measuring stick.
In 1918, Edwin Hubble - armed with this new measuring stick - focused the enormous new Mount Wilson telescope on M31 and found a Cepheid Variable. When he applied Leavitt's ruler, he was able to determine that M31 was further away from us than any other star known at the time.
Indeed, he was able to show that M31 was a completely separate galaxy millions of light years away and millions of years in the past.
From this early beginning, astronomers have pushed back the boundaries of the Universe. We now use the redshift of galaxies - discovered by Hubble - to determine their distance. The more redshifted a galaxy is, the faster it is moving away from us and the further away it is.
As we look at the most distant galaxies, we are also looking back in time to the very childhood of the Universe around us. We can now see galaxies that formed just a few hundred million years after the big bang.
Still, for cosmologists in their quest to understand how everything came to be, a picture of the Universe at the moment of its birth is what they would like to see.
While a picture of the big bang will likely never happen, the Planck space observatory has given us a much better picture of the cosmic dawn. This is the period when nuclei and electrons had finally coalesced into atoms of hydrogen and the first stars and galaxies burst into existence.
Light pervaded the cosmos for the first time putting an end to the true dark ages.
The Planck observatory can see the microwave background left over from the earliest days of the Universe. It constantly bombards us from all directions every moment of every day.
And if our eyes were capable of seeing microwaves, when we looked up at the nighttime sky, we would see the Universe as it was 13.8 billion years in the past.
