Star Light, Star Bright

After 11 Days with no or limited view of the sun and stars, I feel lucky when there is a momentary clear view! I am also reminded why GPS is such an amazing invention...

As long as humans have roamed at night, they have used stars as a guide. When you needed to know what direction to head, you could always look up to find familiar points of light and orient yourself because you knew where they were in the sky. Celestial navigation.

Of the 6,000 stars visible to the naked eye, there are 58 that have been named as “Selected Stars” because their brightness made them the easiest to find in the sky. Many have had names since antiquity, with Babylonian records from 1500 - 1100 BCE cataloguing stars and constellations. Others were named by the Greeks, Romans and Arabs who followed in the Babylonian footsteps of discovery. In modern times, Her Majesty’s Nautical Almanac Office and the US Naval Observatory has had the responsibility for curating the list of Selected Stars.

Stare at the stars long enough, and it’s human nature to find patterns and invent stories. Draw lines between different stars and you can come up with shapes that represent objects in your life, and when those patterns are part of something so vast and mysterious as the universe, then often those shapes are inspired by mythology and religion. The oldest description of constellations as we know them come from a Green poem written around 270 BC, but historians believe that the concept of constellations existed all the way back to the Babylonians. What we do know is that by 150 AD the Greek scientist Ptolemy published a book that identified 1,022 stars arranged into 48 constellations. In the 19 centuries since then we have added 40 more constellations to that list. Of the 88 recognized constellations there are 14 humans, 40 animals, 29 inanimate objects and a handful of more fantastical things. It often takes a bit of imagination to make the leap between the lines that connect those stars to the figures they are supposed to represent. But imagination is as important as taste and touch and sight and hearing, a wonderful thing to have and nourishment in your travel across the earth and under its glittering sky.

A physicist would tell you that the universe is expanding and as a result the planetary objects we can see are all moving away from us. Practically speaking, that is happening on a scale of time and space that does not register in our day to day lives and as a result the stars we see each night are effectively fixed in their locations in space, and depending on where we are located on earth we can see different stars. It’s as if two people were in the same house, but each looking out of a window on the opposite side of the house. The view is different for those two people and likewise, the stars that you can see in the southern hemisphere differ from those that you can see in the northern hemisphere. For the stars that we can see, we also perceive them to move because of earth’s daily rotation, or as the earth loops around the sun through the year and changes its orientation to the sky that surrounds it.

In its simplest form, celestial navigation is as easy as knowing where in the sky a particular star is located. Polaris is to the north. Venus (Yeah, I know. It’s a planet and not a star) will appear in the sky in the West in the evening time, or if it rises later it will appear in east before the sun rises. The constellation Orion can be seen in October, November and December rising in the eastern sky. Long before there were compasses, if you knew the relative direction of a given celestial object in the sky, you could figure out the direction you were heading in.

In its more complicated forms, the stars can not only tell you what direction you are heading, they can also tell you where you are located. Since the stars follow a predictable path through the sky, rising and setting at predictable times, then if you are at a specific location at a specific date then you will know exactly what angle one of the Selected Stars will be at above the horizon for each specific time during that day. To over simplify it, because of the predictability of earth’s rotation on it axis and it revolution around the sun we know where a start will appear in our sky: Specific Selected Star + Date + Time + Location = Angle from Horizon to Star.

When you are at sea, you don’t know your location, but you do know all the other parts of that equation so to figure out your location, work it backwards. Specific Selected Star + Date + Time + Angle from Horizon to Star = Location. Using an instrument called a sextant, measure the angle between the horizon and a Selected Star. Note the exact time and date that you took that measurement – you need to have an accurate watch and Dana Sobel wrote an awesome book called Longitude on the challenge to invent such a clock that would work on the high seas. Then look up that star’s name with its associated angle, time and date in a set of tables that you carry around with you. If you measured a Selected Star in the northern sky such as Polaris those tables can tell you what your latitude is, or the line that you are sitting on that runs east / west. Then if you measure the angle of a Selected Star in the eastern Sky the calculation you run can tell you your longitude, or the line that you are sitting on that runs north / south. Draw those two lines on a map and where the cross is where you are located. Measure even more stars, and you can increase your confidence in your outcome.

Those ancient Babylonians were the first to record the predictability of stars and how that could be used to aid in navigation, and the civilizations that followed who roamed the trackless seas and the deserts – the Greeks, Romans, Arabs, Portuguese, Spanish, British, French and others – built on that knowledge to with their effort to explore the unknown. For the past 25 years or so, GPS has made it easy for any person with a smart phone to be able to find their location on earth, but for hundreds and hundreds of years prior to that the navigators that discovered, explored and ultimately settled the modern world relied upon celestial navigation.

Learning celestial navigation can be a bit complicated and requires some specialized equipment, but learning more about the skies above us is thankfully easier. For astronomers at home, I can offer up two excellent resources. Check out an excellent and easy to use sky map. Set any location that you like location as well as a date and time, and the tool will show you the stars and constellations. Also check out www.stellarium.orgfor a site that allows you to download software onto your PC that turns it into a planetarium. It will require about a gig of memory and use a gig of RAM when running, but it provides an awesome window to the universe.

Sailing sits at the beautiful intersection of practical reality and aching romance. In no place is this better illustrated than with stars, in their ability to guide a boat across trackless oceans as we sail under their diamond beauty scattered across the sky. A camera is poor way for me to convey how beautiful it is out here as I slide through the night, but each night I will post to {website / facebook / twitter} try to give you a view into what I can see. Or what I should be able to see if there were no clouds. I hope you enjoy learning about them as much as I enjoy following them across the ocean.