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Does anybody know the time? Catch a wave
Q:
Is International Standard Time the same as Coordinated Universal Time (UTC)?
(Dave, Dublin, Ireland)
 A twelve-hour analog clock inside the Quai d’ Orsay Museum
(a converted railroad station) in Paris. [Corel Corporation, Canada]
A: No. They both are conventions concerning time but their
purposes differ.
Coordinated Universal Time is a reference time for events. It
is based on and almost identical to the old Greenwich Mean Time (GMT) along 0
degrees longitude (the line that passes through Greenwich in London, England).
We translate the UTC reference time to our local time by adding or subtracting
the hours that correspond to our time zone. For example, (UTC - 6) is USA
Central Standard Time and (UTC + 1) is Central European Standard Time.
UTC provides a reference for calculating time, planet-wide,
for a given instant. If everyone knows that a Moon eclipse starts at 0114 UTC on
Oct. 28, then people everywhere can calculate the instant the eclipse starts —
their time. Kansas City folk will look at the sky at 1914 CST (7:14 p.m.) the
day before (Oct. 27) and the Swiss will look at 0214 (2:14 a.m.) Central
European Standard Time, Oct. 28. Both local times, however, refer to the same
instant — when the eclipse starts.
International Standard Time, on the other hand, is a
particular style of writing the time — local time. The convention is:
hh:mm:ss, where hh is the number of hours past midnight, mm is
the number of minutes past the hour, and ss is the number of seconds past
the minute. So, 23:59:59 is one second before midnight.
The purpose of International Standard Time is to eliminate
ambiguities that can arise with other time notations (like our notorious 12-hour
time) and ease international communications.
For instance, how do we write 00:00:00 and 12:00:00 — so clear
in 24-hour time (midnight and noon) — in 12-hour time? Well, we can try 12:00
a.m./p.m. for noon but that isn’t clear.
"These times are legally 12 noon and 12 midnight," says Robert
Massey, astronomer at Royal Observatory Greenwich. "Neither should be given am
or pm as, on the average, the Sun is smack on the meridian at those times."
Handily enough, we can write UTC using the International
Standard Time notation. Then, the convention is to append Z to the time
so the reader knows the time isn’t local time but rather UTC. For example,
23:59:59Z is one second before midnight at Greenwich in London.
Z denotes UTC time because it’s the
time at Zero degrees of longitude (London). UTC time is also called
"Zulu" time, referring again to the "Z" but in the international phonetic radio
alphabet.
By the way, UTC is a compromise abbreviation for the English
"Coordinated Universal Time (CUT)" and the French "Temps Universel Coordonné
(TUC)."
Further Reading:
Royal Observatory Greenwich: Timekeeping by the Earth, Moon, and Sun
Wikipedia Encyclopedia: Coordinated Universal Time
University of Cambridge: International Standard Date and Time Notation by Markus
Kuhn
Helis.com:
The International Radio Alphabet
Q:
What’s the longest radio wave (and, therefore, has the lowest frequency)? Which
equation relates the wavelength, frequency, and speed of an electromagnetic
wave? (Melva, Dallas, Texas)
 A wave ends its travel [NOAA]
A: The longest radio wave that we've detected is a whopping 19
million miles (0.01 Hertz). That’s 100 million times longer than waves in the
middle of the AM radio bandwidth. But, there is no theoretical limit.
Micro pulsations in the Earth’s magnetic field cause these
extremely low frequency waves.
Now for your second question: The speed of anything is the
distance the thing travels divided by the time it takes to travel that distance.
So, how fast a wave travels past a given point is simply the length between
successive crests (or troughs) divided by the time it takes for two successive
crests to pass the point. That time is called the period of the wave and
is defined to be 1 / frequency.
Thus
wave speed = (wavelength) / (period)
and since 1 / period = frequency, we obtain
wave speed = (wavelength ) x
( frequency)
We can, however, nail down electromagnetic wave speed
completely. As Einstein pointed out, the speed of light in a vacuum always
travels at 186,000 miles/second (300 000 km/s). We use c as the symbol
for the speed of light.
So, for electromagnetic radiation, the equation becomes:
c = wavelength x frequency.
This useful equation allows us to calculate wavelength if we
know frequency and visa versa. Thus, a 0.01-Hertz wave has a wavelength of 30
million kilometers — a super long wave.
Further Reading:
The
geodynamo by Gary Glatzmaier
The Physics Factbook edited by Glen Elert: Wavelengths of the longest
electromagnetic waves
NASA: The electromagnetic spectrum
(Answered Nov. 19, 2004)
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