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I know where, but when?

Timing can make or break an observing session, but don’t let converting to Universal Time bog you down.
ChapleGlenn
Noting the time that a predicted astronomical event will occur can be problematic. If I told you that a comet is predicted to impact Jupiter at 9:00 P.M. on May 1, you’d rightly ask, “9:00 P.M. where?” In a world ajumble with some two dozen time zones, we need a standard way to express the time of astronomical events.

To this end, astronomers use Universal Time, abbreviated UT. To avoid the confusion between A.M. and P.M. hours, UT utilizes a 24-hour “military time” clock, and it is based on the time at the 0° longitude meridian in Greenwich, England.

For any place on Earth outside this zone, you have to make a correction. Here’s how it works for me. I live in the Eastern time zone. During Eastern Daylight Time (EDT), which runs from the second Sunday in March through the first Sunday in November, my clock is four hours behind Universal Time. The rest of the year when clocks “fall back” to Eastern Standard Time (EST), I subtract five hours from UT.

Unfortunately, there will be no comet/Jupiter collision, but the big planet arrives at opposition this month. According to the 2018 Observer’s Handbook — an annual publication put out by the Royal Astronomical Society of Canada — the precise time of opposition is May 9 at 1h00m UT. That translates to May 8 at 9:00 P.M. EST.

I don’t plan to mark that date and time on my calendar because Jupiter will be a readily seen telescopic sight for months to come, but other Jupiter-related events require precise times, too. For instance, if the skies are clear on the evening of the 8th, I may want to take out my telescope and try for the Great Red Spot (GRS). Will it be visible then?

To find out, I’ll need to know the time when the GRS will be near the center of the side of Jupiter’s disk that faces us. I turn to Project Pluto (specifically, www.projectpluto.com/jeve_grs.htm), which supplies transit times for the GRS. A transit is predicted at 1h17m UT on May 9, which corresponds to 9:17 P.M. EDT on May 8. This is an hour after sunset, so Jupiter should be comfortably positioned above the eastern horizon.

ASYGC0518
Astronomy: Roen Kelly after Bruce Macevoy; Dylan O’Donnell
What about your time zone and the correction you need to make to convert to UT? Astronomy simplifies the process by posting times in Eastern Standard (or Daylight) Time. You can also refer to a chart and article on EarthSky (earthsky.org/astronomy-essentials/universal-time). The article refers to Universal Coordinated Time (UTC), which, for all practical purposes, is identical to Universal Time.

Last January, I described three simple ways to determine the true field of view (TFOV) of any eyepiece/telescope combination. Mike Bertin, of Orange County Astronomers in California, offers another method for those of you who own a go-to scope that displays altitude and azimuth. The beauty of this method is that it works during daylight by using a distant terrestrial target, like the top of a light tower.

He writes, “I level the scope — eyeballing it is OK, but I usually use a small level that I have mounted on top of the optical tube. Next, I move the telescope in azimuth so the object is at one edge of the field of view, and I write down the azimuth value. Then I move the telescope so the object is at the other edge of the field of view. I write down the new azimuth value, and by taking the difference, I have the TFOV in azimuth.”

Bertin then double-checks his result by repeating the process in altitude. “I once again center on my reference object with the scope still level. I move the telescope so the reference object is at the bottom of the FOV, and I note the altitude value. Then I move the telescope so the object is at the top of the FOV, and I write down the new altitude value. The difference gives me the true field of view in altitude. All of this takes less than 5 minutes.”

Al Nagler, a pioneer in the production of ultra-wide-field eyepieces, notes that the apparent field of view divided by magnification does not give an accurate TFOV because of eyepiece focal length and distortion variations and tolerances.

He notes, “The actual calculated true field of ANY eyepiece is its field stop diameter divided by the telescope focal length, times 57.3°.” Field stop data for all Tele Vue eyepieces are published on the company’s website. If you’re not sure of the field stop diameter for a different brand eyepiece, the methods I mentioned in the January column will suffice.

Do you have questions, comments, or suggestions of your own? Email me at gchaple@hotmail.com. Next month: The missing Messier object. Clear skies!

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