Dear Mr. Sir- Hi, Do you know how many exact hours,
minutes, and seconds there are in a day? My Science teacher says
there are actually a little less then exactly 24 hours, but they
rounded it off so it'd be easier for people. Please w/b and tell me
what you think. I like your page!! It is interesting!!! Thanks
again, Sincerly, Courtney
Your teacher is right. The official
scientific definition of a day is that it has the length of 86,400
SI (Systeme International) seconds. 86400 = 60 seconds in a minute
times 60 minutes in an hour times 24 hours. For most of us a day is
average length of time from noon one day to noon the next over the
I said average because sometimes the sun is fast or slow.
You might check the terms "Equation of Time", the
"Analemma" which is a figure 8 diagram printed on some old
earth globes in the expanse of the Pacific Ocean, and "Sundial"
in an encyclopedia for more information on the sun being fast or
Anyway the day interval is for timekeeping. When scientists
developed super accurate atomic clocks, they found that the earth's
rotation wasn't constant. This posed a problem. If you want to get
up and go to school the same time every day you'd better have your
clocks geared to the sun. But if you were plotting planet or
spacecraft orbits, which had nothing to do with the steadiness of
the earth's rotation, you need a constant clock.
So how to handle the problem? At first the folks at the
National Bureau of Standards who distributed time tweaked the length
of the second to make the day come out, But there were other
timekeeping systems that could detect the change, and it could mess
them up. So scientists from all over the world got together on this
and other matters and set the basic unit of time to be a second, at
the length it was about the year 1900. They then defined it in super
accurate terms on the atomic scale as: (are you ready?)
"9,192,631,770 cycles of radiation cooresponding to the
transition between two hyperfine levels of the ground state of
cesium 133". Even I'm not sure what all that actually means.
But I do know that a scientist on the earth, or in the
Andromeda Galaxy would know how long a second was. And a day
has 86,400 of them.
That means that as the earth's rotation slows down, and it
does principally due to the drag of the tides raised by the moon and
sun, a second is inserted into the time stream occasionally to keep
our super accurate clocks within a second of earth's rotation with
respect to the sun. These "leap seconds" can be added only
two times a year, The last second of June 30th and the last second
of December 31st Coordinated Universal Time or UTC. UTC is what is
served by all the time services of the world. If you want to hear a
time service, set a short wave radio to 5 or 10 MHz to the radio
station WWV. Just before of after the minute mark double ticks tell
you if the UTC clock is so many tenths of a second slower or faster
than the earth's rotation.
There's another definition of a day that astronomers use.
That's the sidereal day, which is earth's rotation rate with respect
to the stars, rather than the sun. It's kept at observatories around
the world by sidereal clocks which gain 3 minutes and 56 seconds
over normal clocks each day. In fact they gain exactly one day every
year. They are used to locate the observatory with respect to the
I hope this answers your question.
Wishing you clear skies.
Why aren't day and
night really equal on the equinox?
Message text written by Joanne: I was wondering why
the actual fall equinox(12 hours of day and 12 hours of night)
occurs a couple of days after the date given and in the spring,
equinox occurs a couple of days earlier.
The reason the date in which the sun is actually out exactly
12 hours is different than the equinox date is due to two factors.
The definition of instant of sunrise and sunset, and the fact that
the earth has an atmosphere. The definition of sunrise is that
instant when the top of the sun touches the horizon and first
appears. The definition of sunset is the instant the top part of the
For the equinox to give a true equal night and day the
definition of sunrise and sunset would have to state that the event
occurred when the center of the sun crossed the horizon. Also the
earth must not have an atmosphere. The atmosphere causes light to
bend a bit when traveling through it. This is especially pronounced
when the object is near the horizon, and makes the object appear
higher than it actually is. This bending is called refraction.
Here is the wild part: The sun is half a degree in diameter,
and a quarter of a degree in radius. So if there was no atmosphere
at the instant of sunrise and sunset the center of the sun would be
a quarter of a degree below the horizon. Add atmospheric refraction
of a bit more than half a degree, and the center of the sun is
officially nearly twice its diameter below the horizon at sunrise
To check this out, let's do a simple calculation. The sun,
as I said, is half a degree in diameter. Also the earth's rotation
rate is one degree in 4 minutes. If the sun appears to set when its
center is nearly one degree below the horizon, That retards sunset
by nearly 4 minutes. Likewise it speeds sunrise by the same amount.
So one would expect the sun to be up close to 12 hours 8 minutes on
the date of an equinox. My calculations for September 23rd show the
sun to be out about 12 hours 7 minutes. That's pretty close for ball
I hope that helps.
for a horoscope
Message text written by "Heidi xxxxxx"
I hope that you can help me..... I am just learn astrology
and want to try and make up charts for my kids and myself. I have a
problem trying to work out the longitude and latitude for south
africa. (time zones, etc, etc.....)
Asking an astronomer about astrology is like
asking a botanist to read tea leaves. neither of us do that.
that as it may. To get longitudes and latitudes for places in South
Africa, you will need maps or a world atlas. You may get time zone
information from the same sources. I know Johannesburg is at UT + 2
Thoughts on Equinox Egg Standing:
Around March of 1999 I got more emails than any
other time asking me when the exact time of the equinox was. and what
I thought about the ability of standing a raw egg on end them. Here's
What I know:
I heard once some years
ago in an interview on National Public
Radio'sAll Things Considered that someone was able to
stand raw eggs on end near the exact time of the Vernal (spring)
Equinox and Autumnal Equinox.
I've heard other sketchy
I've heard no scientific evidence for or against it.
What I surmise:
What seems to be implied
is that supposedly raw eggs can't be stood on end any other time.
What seems to be suggested to me is that some kind of
alignment of the sun and earth at an equinox allows the egg to keep
What I think:
In order for such an
equinox alignment to work one would have to be on the earth's
equator where the equinox occurred when the sun was at the zenith.
Actually the moon has a greater tidal effect than the sun, so should
be either be at the midpoint of a central lunar eclipse or in the
middle of a solar eclipse at the same instant. It would also be
helpful it the two planets with the greatest tidal effect on the
earth were at the optimal position: Venus at inferior conjunction
with the sun and Jupiter at opposition from the sun at that same
instant. At any other location on the earth there will be lateral
forces to topple the egg from the sun, moon etc., even at an
equinox. And I haven't even mentioned the coriolis force due to the
So what could it be? I'm
betting on the Placebo Effect. That is, if one believes something
can be done, it has a better chance of happening. Does anyone put in
as much effort to stand an egg on end at times other than an
equinox? How clean, level and smooth is the surface?
That's what I think. I
don't think funds would be well spent trying to scientifically prove
this one way or another. However one should always be skeptical of
all claims, even mine.
Accuracy and Looking Out of the Milky Way
Thanks for your kind words. Here goes with your questions:
Q: Is there any source listing magnitudes and distances of
stars that is recognized as definitive? I ask because, for any given
star, I can look up magnitude and distance in a half-dozen different
reference books and come up with a half dozen different answers.
Someti mes, and this is especially true with distance, the answers
will differ by as much as 200 light years.
A: Measuring star distances is a very tricky
endeavor. Stars less than about 200 light years can be measured by
parallax or triangulation using the radius of the earth's orbit as
one side of a long triangle. Errors from one measurement to another
can be great, and astronomers are always refining those distances.
Distances beyond that are extensions of the stars we can measure by
triangulation. The Hipparcos satellite in the last few years has
revolutionized parallax measurements, so we can expect more accuracy
as time goes on. In general the newer the source the more accurate
(2) I'm afraid this question is REALLY going to
demonstrate my ignorance, but here goes. Most guides to the night sky
talk about the fact that we're looking towards the bulge in the
center of the galaxy, no matter what the time of year. This just
doesn't make sense to me. I keep trying to picture it, but it seems
that i n our yearly orbit around the Sun, there'd be a period of time
where we'd be looking away from the bulge. I came across an astronomy
website where it mentioned that when you look towards the center of
Orion, you're looking out beyond the galaxy -- is this true?
A: The center of the Milky Way galaxy and its
central bulge is located in the direction of the constellation
Sagittarius, near the spout of the Teapot pattern. (Sagittarius
appears to us more like a teapot rather than a centaur with bow and
arrow that the ancients saw). Orion is just about opposite that area,
but we are still looking through the Milky Way band, since we are
located within the Milky Way ourselves. It's easy to look outside the
Milky Way by looking away from the milky band. There in telescopes
can be spotted other galaxies like our Milky Way. One can even be
seen with the unaided eye, the Great Andromeda Galaxy. I hope that
answers your questions.
Frequently asked and
quickly answered questions
Have you ever seen a UFO?
Lets put it this way: I've never seen anything that couldn't
be explained by something that belonged there.
Do you believe in UFOs?
That UFOs are what the initials represent, namely something
that appears to be flying that can't be identified: Yes. That
they're alien spaceships: No. The state of knowledge of things
astronomical by most folks is unfortunately abysmal. How can someone
determine what doesn't belong in the sky, if he or she doesn't know,
or can identify what does belong there? Part of the reason for the
Ephemeris site is to help folks learn more about what is really and
truly in the sky.
Do you believe in life on other
Why not, it's a big universe. And why confine life only to
Do you believe in astrology?
Do you believe in Creationism or
I believe* that the universe and everything in it started at
the Big Bang. To me that was the moment of creation. The nature of
the universe is such that everything we see now evolved from that. I
don't believe the account in Genesis is to be taken literally. Since
we can see objects in the heavens, even with the unaided eye, whose
light has traveled to us for longer than the 6,000 to 10,000 years
of the literally interpreted age of Genesis, we would have to be
seeing light from objects which didn't exist when the light would
have been sent out to us. If that were true astronomers might as
well pack up the telescopes and go home. The observed universe
wouldn't be real. And if God created the universe 10,000 years ago
with an apparent age of billions of years. Why couldn't he create
the universe next Thursday. Then we'd be the invented past to a
universe that's yet to be. Like Einstein, I don't think God is
* The word believe on my part in the
context of science is not a belief based on faith. Should a theory
or explanation make more sense to me than the Big Bang I'll switch