I’m observing remotely from Ithaca for the next few nights and thought I would take the opportunity to write about topics that I find relevant to observing. One of the questions I ask myself a lot while observing is: What time is it? Often this is a question of frustration, especially around four in the morning (which is when I have the most trouble focusing and staying awake). Other times, though, it is a relevant question for the observing. Mostly this is because there are a lot of different times that you need to keep track of while observing.

First, there’s the time zone you live in. For remote observing this time zone remains important because you are still operating in it. When traveling to the telescope this is a less important time, although it does affect your body’s .adjustment to the night observing schedule.

The second time of importance is the time zone where the telescope is located. When I look at an observing schedule for Arecibo, I need to remember that the times listed are in AST (which I like to think of as Arecibo Standard Time even though I know it’s Atlantic Standard Time) and not my local time zone (EST or EDT depending on the time of the year). The difference between the telescope time and my local time can lead to me being extremely paranoid and double- and triple-checking start times. If I’m observing at an optical telescope, I typically know sunrise and sunset in local time and schedule accordingly. It can also be quite confusing to switch between working on my laptop which is on my local time and the computers used for observing that show the telescope time.

There’s a third time zone to worry about, too: Universal Time (UTC). Often, astronomers choose to record time stamps for data in UTC to avoid the confusion of time zones and daylight savings time. I don’t use this time for the ALFALFA observations but have used it when observing at optical telescopes. Typically, I don’t have to worry about thinking in this time zone, though; I only need to occasionally write the time down from the computer screen.

Three time zones manages to confuse me a bit as it is but are fairly manageable. After all, it’s just adding/subtracting a certain number of hours. Annoyingly, the number of hours changes with daylight savings time, making life that much more interesting in spring and fall as you try to remember the change. (I tend to be a little paranoid when it comes to observing.) However, there is another type of time important in astronomy that is not as simple. That time is LST: Local Sidereal Time.

Essentially, this new type of time is related to the coordinate system used for stars and astronomical objects (hence the sidereal), rather than being based on a solar noon. I found the concept of LST confusing at first, but it makes observing much easier once you understand it, so I’m going to attempt to explain it. The first step to understanding LST is knowing a little about the coordinate system used to describe the sky. Typically, the sky is pictured as a “celestial sphere” where all the objects visible in the sky are projected onto a sphere. This sphere is defined by two coordinates: right ascension (RA) and declination (DEC). Right ascension is the analog of longitude on the Earth and is marked in hours from 0h to 24h. Declination is the equivalent of latitude and runs from -90 degrees to +90 degrees. A schematic of the celestial sphere is shown below.

Celestial Sphere

Celestial Sphere


As the Earth rotates within the fixed celestial sphere, the ranges of right ascensions visible will change as objects rise and set over the horizons. LST is a way of conveying what astronomical objects are visible at a given time. The current LST refers to the RA that is currently at zenith (highest point in the sky). Thus, if the LST is 08h 44m 14s currently, an object at that RA would be at its highest point in the sky for the night. Since the coordinates of astronomical objects are normally given in RA and DEC, knowing the LST helps one know immediately if an object is observable. For the LST I just mentioned, observing an object with a RA of 08:05:36 would make perfect sense, but if the RA were 04:21:43, the object would be near setting (almost four hours from its highest point) and I probably wouldn’t want to observe it. If the RA were 20:21:04, the object would be on the other side of the Earth (about twelve hours away from zenith) and unobservable.

So, the sidereal in LST comes from the fact that the time is determined by the coordinate system defined for stars (and other astronomical objects). The local is also quite important. The LST at Arecibo is quite different than that at Mauna Kea, Hawaii. An object that is overhead in Puerto Rico would not have yet risen for Hawaii. That’s one of the beauties of LST; it tells you exactly what is observable for your location. Once you get the trick of it, LST is the most useful time for observing as it gives you an immediate sense of where an object in the sky is located. Is it near zenith? If the RA is near the LST it is. Is it still rising? LST increases throughout the night, so if the RA is greater than the current LST, the object is still rising and can be observed later. Has the object already passed by? If the RA is much smaller (typically by about four hours) than the LST you have probably missed your chance to observe the object that night and should schedule it for tomorrow.

Of course, LST lacks intuition in the sense that I can’t say, “Oh, it’s about 7:30 LST”. I let the computer tell me what the LST is currently. A large part of the reason that LST is not intuitive is because it doesn’t track the solar time we are used to. If the LST at midnight tonight is 07h 30m 00s, in six months the LST at the same solar time would be 19h 30m 00s. If you think about the difference in constellations from the summer to winter months, this difference makes sense. We know that we see different constellations overhead at night at different times of the year. This is the result of about a four minute difference between a day as defined by the sidereal and solar systems. While this difference is understandable, it does mean that if you ask me at any given time what the LST is, I’m unlikely to know unless I happen to be observing.

You can see that the question “What time is it?” isn’t always the easiest question to answer. Hopefully, you’ve learned about some of the different answers you can have. For me, the current times are:
Local: 12:50 AM
Arecibo: 1:50 AM
LST: 09h 18m 35s
The real time I care about? It’s about five hours until observing is done. I’m feeling pretty awake right now, but in about two-three hours I probably won’t be feeling as alert.

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