I am so behind! A lot of stuff has been going on for the past week, I’ve actually generated quite a backlog of things to write about – but finding the time to write and get it out there has been a challenge of late.
This week’s astronomy tidbit is a little different… I ran across the coolest sundial design I’ve ever seen today, and so I just had to share. I’ve talked about DIY sundials a bit in the past, as a quick and dirty way to mark time in the backcountry. I made a point during that discussion to not really worry about the “hour”, per se, but instead use this technique to look at trends: mid-day, mid-afternoon, etc. One of the reasons I shied away from adding any precision, aside from questionable value in it, is that “noon” is not a consistent thing over the course of a year. I mean, the sun does what it does, and of course it always has a highest point in the sky on a given day, but that doesn’t really fit well with our orderly definition of hours, minutes, even days.
The reason is all geometry. On the one hand, Earth’s axis is tilted 23.5 degrees with respect to its orbital plane – and we’ve noticed how that effect pushes the ecliptic lower in the summer night sky, and higher in the winter. The sun does the opposite – high in the summer, low in the winter (i.e. why we have those seasons in the first place). So the sun is moving north-south over the course of the year.
The other factor moving the sun around, with respect to the clock, is the eccentricity (non-circularness) of our orbit. Since our orbit is an ellipse, the Earth moves in its path slowly at its farthest distance from the sun (aka the aphelion, this year that happened on July 3). Conversely we’re moving fastest at our closest point (perihelion, which occurred on January 4). While those speeds are changing, and the portion of our orbit we cover in a day changes, our rotational rate is essentially constant – which means the relative position of the sun from one Earth rotation to the next is not the same in winter (near perihelion) as it is in summer (aphelion). This causes the sun to drift east-west at a given time of day as our orbital speed changes.
The result of these two factors is the solar analemma. This figure-8 like pattern is the path the sun takes in the sky, IF you were to look at it at the same time of day, every day, for a year. This pattern is even printed on most globes. Fortunately some creative photographers have captured this, and the result is shown here.
If this is the pattern, shown in north-south, east-west deviation from mean as a function of calendar date, then the same data can be shown as a set of 2-D graphs depicting the deviation of elevation and azimuth as a function of time – or by calendar day, or month.
So, back to the sundial… most sundials are set up to negate the north-south movement of the seasons. The gnomon, also known as the pointy bit that casts a shadow, points north at an angle equal to the latitude at its location. Thus, it’s parallel to Earth’s axis, and therefore perpendicular to the average path of the sun. North-south deviations are taken care of by the shadow just getting shorter or longer. Pretty simple so far.
But THIS sundial has an extra feature – an inner arc to project the shadow on a detailed time scale that ALSO captures the east-west azimuth shift as a function of time of year! Looking at the straight line grid, the shadow edge falls between 1500 and 1515 – lets call it about 1508 (3:08pm). But the curves show that 1500 isn’t really a straight line, and that in the latter half of July (each horizontal line is the first of the month, with the bottom line Jan 1. So count months up from the bottom), the shadow hits 1500, and 1530, earlier than the straight line average. Using the curves, the shadow is closer to halfway between the two times – 1515 (3:15pm). The actual time-stamp on this photo is 16:14:43 – 4:14 and 43 seconds. But it’s Daylight Savings Time here, so solar time is just shy of (…drumroll please…) 3:15pm! Not too shabby!
There are some limitations to this method of course – it’s pretty complicated to make accurately, it only works when the sun is shining, etc. But getting precision like this, within a minute or two, on a tool with zero moving parts is pretty darned cool. And all because of some careful astronomical observations and record keeping!
If you’re interested, this sundial sits by the waterfront on the campus of St. Mary’s College of Maryland, in St. Mary’s City. The plaque indicates it was designed by Everett Merritt, and built by John Allard, Robert Abell, Barry Merritt, Bruce Merritt, and Mike Ironmonger in 1979. Thanks, guys!
Get Out There!