Astronomers love to draw imaginary lines in the sky. Without fixed landmarks, it is the only way we can reliably navigate the celestial sphere. The ecliptic and the equator, declination and right ascension – these are all attempts at breaking up the night sky in to manageable chunks. The meridian is yet another of these lines and is closely tied to how we track time and how professional astronomers plan their observations.
To trace out the meridian yourself, point your arm north. Now swing your arm up directly over your head and then down again to the south. You’ve just drawn a line dividing the sky neatly into eastern and western halves. Facing south, everything to the left of the line is rising; everything to the right is setting. The meridian is different from celestial markers like the ecliptic because it is completely local. One observer’s meridian is different from someone else’s. Only observers standing at exactly the same longitude will mark out the same meridian.
As a star crosses the meridian, it reaches its highest point on its nightly journey – a moment known as “upper culmination“. Knowing a star’s time of upper culmination is essential for an astronomer trying to get the most out of her time at a telescope.
Light from a distant star or galaxy can travel for thousands or even millions of light-years with out being perturbed…and then in the last nanoseconds of its journey become irreversibly entangled in our atmosphere. Much like looking at oncoming headlights through a thick fog, the air we breath dims starlight. Atmospheric turbulence also causes the stars to twinkle. This effect can be lovely to look at but can be rather frustrating for the astronomer trying to make a detailed measurement of some celestial object.
The more air one looks through, the greater both of these effects become. You can see this effect for yourself by looking at a bright star near the horizon and comparing it to one high in the sky. At this time of year, the stars Sirius and Regulus are good stars for comparison. For those living in mid-northern latitudes, bright blue Sirius sits low in the western sky shortly after sunset while Regulus is almost directly overhead, sitting close to the red speck of light that is Mars. Sirius dances and twinkles with all the colors of the rainbow while Regulus burns steady and bright, only a slight shimmer revealing the sixty miles of air sitting on top of your head. Starlight coming from low in the sky must pass through more air than light coming straight down. The twinkling, therefore, becomes more apparent when stars are close to rising or setting. And while it’s hard to discern with unaided eyes, the light is considerably dimmed as well.
For this reason, astronomers try to time their observations for when the star, planet, or galaxy of interest is crossing the local meridian. This ensures that their target is as high in the sky as it can be and therefore maximizes the amount of light reaching the telescope.
But the meridian has an impact well beyond the needs of professional astronomers. It is also central to how we mark time. The duration between successive crossings of the meridian by the Sun is how we define one 24 hour day. Using the Sun’s meridian crossing greatly simplifies time keeping – when the Sun is sitting at its highest point in the sky, shadows on the ground are shortest. Marking noon therefore becomes a simple matter of watching shadow lengths.
One downside of this is that any two towns lying on different lines of longitude will mark noon at different times. This may seem obvious when comparing widely separated cities like New York and London. But what about the difference between London and Oxford? Or New York and Trenton? These are cities separated by only about sixty miles and yet their noons are different.
For most of history, this is exactly what happened. To people that rarely left their home town, the fact that every town and city had their own local time was irrelevant. It only became a problem with the advent of rail travel. Suddenly, Boston and New York were reachable after mere hours in a train and the problem of synchronizing clocks for the purpose of scheduling quickly became a nightmare. Different rail companies took to using their own time – which made connecting from one rail line to another in a distant city an adventure in time table translating.
The terrible burden and confusion this introduced directly led to the creation of time zones. Now, towns within agreed upon boundaries of – nominally – constant longitude would all use a common time known as the “zone time”. Approximately, this fell to synchronizing the entire zone to local noon measured somewhere in the middle of the time zone. Noon in your town may therefore no longer happen at exactly the moment of the Sun’s upper culmination. Depending on your location within the zone, it could be as much thirty minutes before or after the local solar meridian crossing.
Keeping time with the Sun actually becomes even more complicated than this. Because the Earth’s speed around the Sun is not constant over the course of the year, the duration between successive solar culminations varies. A day tied strictly to solar noon is longer in July, when the Earth is farthest from the Sun and therefore traveling slowest, than it is in January when the Earth speeds through its point of closest approach. To keep a constant day, zone time is therefore not tied to apparent solar noon but what is called “mean solar noon” – or where the Sun would be if the Earth orbited the Sun at a constant speed. The difference is not insignificant – apparent solar noon can be ahead of mean noon by 16 minutes (in November) or behind by 14 minutes (in February).
Something as esoteric as maximizing telescope efficiency and something as basic as defining a day – probably not two things you’d put together. But they share the common thread of determining the moment that a celestial object – be it the Sun or a distant galaxy – reaches its highest point in its daily trek across the celestial sphere. Some concepts in astronomy are pretty far removed from our daily lives. But an idea as fundamental as noon starts with a simple line bisecting the sky.