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70 ASTRONOMY • MAY 2018

N

oting the time

that a predicted

astronomical event

will occur can be

problematic. If I

told you that a comet is pre-

dicted 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 astro-

nomical 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 out-

side 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 opposi-

tion this month. According to

the 2018 Observer’s Handbook

— an annual publication put

out by the Royal Astronomical

Society of Canada — the pre-

cise 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

OBSERVINGBASICS

BY GLENN CHAPLE

I know where,

but when?

Timing can make or break an observing session, but

don’t let converting to Universal Time bog you down.

Last January, I described

three simple ways to determine

the true field of view (TFOV)

of any eyepiece/telescope com-

bination. 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 usu-

ally 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 tak-

ing the difference, I have the

TFOV in azimuth.”

Bertin then double-checks

his result by repeating the pro-

cess 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 tele-

scope 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 appar-

ent field of view divided by

magnification does not give an

accurate TFOV because of eye-

piece focal length and distor-

tion variations and tolerances.

He notes, “The actual

calculated true field of ANY

eyepiece is its field stop diam-

eter 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 men-

tioned in the January column

will suffice.

Do you have questions,

comments, or suggestions of

your own? Email me at

[email protected]. Next

month: The missing Messier

object. Clear skies!

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 (specifi-

cally, http://www.projectpluto.com/

jeve_grs.htm), which supplies

transit times for the GRS. A

transit is predicted at 1h17m

UT on May 9, which corre-

sponds to 9:17 P. M. EDT on May

8. This is an hour after sunset,
   so Jupiter should be comfort-
   ably positioned above the east-
   ern horizon.
   What about your time zone
   and the correction you need to
   make to convert to UT?
   Astronomy simplifies the pro-
   cess 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 prac-
   tical purposes, is identical to
   Universal Time.

BROWSE THE “OBSERVING BASICS” ARCHIVE AT http://www.Astronomy.com/Chaple.

ASTRONOMY

: ROEN KELLY AFTER BRUCE MACEVOY; DYLAN O’DONNELL

Glenn Chaple has been an

avid observer since a friend

showed him Saturn through a

small backyard scope in 1963.

In a world ajumble with some two dozen

time zones, we need a standard way to express

the time of astronomical events.