Clocks and Time Zones

Waiting for some more bad weather to clear, so let’s talk about time …

There are a handful of different time zones that are relevant to this project:

Local Time: Right now, local time for me is Eastern Standard Time. This is the time zone that I use for planning telescope observations. I can easily look up the time of sunset and time of sunrise in the newspaper; both are most easily found as EST. The clock in my bedroom that tells me when to wake up, go outside, and shut down the telescope is set to local time.

Coordinated Universal Time (UTC): UTC is the “master” time that controls my computers. Each computer (including the telescope mount) is synchronized to UTC time to within a millisecond or so. (Right now, EST is 5 hours earlier than UTC.) Each image includes the UTC time that the camera shutter opened. (Although the time that I actually use when I make my plots is the time of the midpoint of the exposure: halfway between the time that the shutter opened and the time that the shutter closed.)

Julian Days: Astronomers are fundamentally lazy, and will invent things in order to avoid work. One of the unpleasant tasks that astronomers like to avoid is answering a question such as the following:

How long is it between 7am (UTC) on January 14, 2018 and 9pm (UTC) on March 11, 2018?

Yuk. And an additional yuk because you have to figure out whether there’s a February 29 leap day in there somewhere.

And so astronomers invented “Julian Days”. The Julian Day calendar started on January 1, 4713BC, and has been counting the days ever since, one day at a time. I am drafting this post on Julian Day 2,458,861. Using Julian Days makes counting intervals really easy.

Further, astronomers will frequently use decimal parts of the Julian Day to measure time within the day. So, for example, astronomers use a fractional part of .000 to refer to noon. So, today at noon is Julian Day 2,458,861.000. And, today at midnight will be Julian Day 2,458,861.500. The time of the first image that I grabbed of KIC 9832227 is 2,458,814.51415.

However, when I go to calculate the orbit time of KIC 9832227, I don’t use UTC or Julian Day directly, because it doesn’t take into account the speed of light. As far back as Galileo, astronomers noticed that the motion of things in the sky (Galileo noticed it with the motion of the moons of Jupiter) sometimes seems to happen up to 8 minutes earlier than expected and sometimes seems to happen up to 8 minutes later than expected. This led to an early understanding that the radius of the Earth’s orbit around the sun is about 8 “light-minutes”. When the Earth is close to the object being observed, light travels some 93 million miles less than when the Earth is at the halfway point in its orbit, and it takes about 8 minutes for light to travel that distance.

And so astronomers “invented” something called “heliocentric time.” This is the time that an observer working at the center of the sun (and using a clock synchronized to UTC) would observe an event. It’s simply UTC with up to 8 minutes added or subtracted depending on which direction you’re looking. You would think that this would compensate for the 8 minute speed-of-light problem, but it doesn’t quite work…

Instead, I’m using a time called Barycentric Dynamical Time (BJD_TDB). This is very close to heliocentric time, but it acknowledges that the Earth doesn’t orbit around the center of the sun. Instead, the Earth orbits around the solar system’s barycenter: the effective center of mass of the entire solar system. The barycenter is offset from the center of the sun, mostly due to Jupiter and Saturn. (The Wikipedia article on barycenter is pretty cool, with animations.) And so Barycentric Dynamical Time takes the speed of light into account based on what an observer located at the solar system’s barycenter would see. This is the time that I actually use when calculating the KIC 9832227 orbital characteristics.

And that brings us to the final “timezone” …

Orbit Time: This is an odd clock; it measures time in the KIC 9832227 orbit. Orbit Time goes from 0 to 10 hours 59 minutes 26.7 seconds (which is how long it takes the two stars in KIC 9832227 to return to the exact same spot that they had at Orbit Time zero), and then resets back to zero. Traditionally, astronomers set the zero of the Orbit Time clock at the midpoint of the deepest minimum in the binary star’s lightcurve. Since I didn’t know when that was when we started observing KIC 9832227, I arbitrarily set Orbit Time to zero at the midpoint of the very first exposure that I made of KIC 9832227 (as measured in Barycentric Dynamical Time). At some point in the future, maybe, I’ll reset my definition of Orbit Time to coincide with that minimum, but I don’t yet have a reason to do so. (And because I’m basically lazy, I’m going to do nothing until I’m forced to make a change.) This Orbit Time is the time that I’ve been using in the Phase Diagram plots that I’ve shown in prior posts.