Frequently Asked Questions

Presented here are some questions on astronomy that have been emailed to me, along with the answers I gave. If you have a question email it to



Length of Day

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,
Hi 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 year.
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 slow.
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 stars.
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.
Hi Joanne,
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 sun disappears.
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 and sunset!
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 park calculations.
I hope that helps.
Bob Moler

Location information 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.

Be 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 hours.

Bob Moler

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 my response:

What I know:

What I surmise:

What I think:

What do you think?

Star Distance Accuracy and Looking Out of the Milky Way

Hi Catherine,

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 the distance.

(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 planets?
Why not, it's a big universe. And why confine life only to planets?
Do you believe in astrology?
Do you believe in Creationism or Evolution?
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 malicious.

* 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 my allegiance.

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Updated 03/30/2005