The varmint was right!

On February 2, Punxsutawney Phil (the groundhog) predicted six more weeks of winter. And, by gum, he was right! It's now six-and-a-half weeks later, and spring has sprung (at least in the northern hemisphere). Today the sun will appear to move north of the equator, where it will stay for the next six months. At the north pole, the sun is rising, and at the south pole, the sun is set (stranding scientists at the South Pole Station until their spring comes).

Tomorrow is the full moon. In the Christian tradition, the first Sunday following the first full moon of spring is the Easter holiday (which explains why Easter is so early this year -- the full moon comes only a day after the start of spring). The Easter holiday remains one of the western calendar's few remaining links to lunar calendars (calendars based on the moon's 29-day cycle through its phases). Lunar calendars are still in popular use throughout the world -- the Islamic calendar, Jewish calendar, and Chinese calendar (among numerous others) are all based primarily on the moon's phase. But since the moon's phases don't match up exactly with the solar year (the length of time it takes the Earth to orbit the sun once, and the basis of the Gregorian calendar we use in everyday life), lunar calendars are a bit odd to the uninitiated, requiring the occasional extra month or something similar to bring the two back in line.

Astronomer's observations are often based on the lunar cycle. The full moon is very bright -- you will notice that you can see fewer stars in the sky when the moon is almost full as opposed to when the moon is absent from the sky. The faintest stars are hidden by the moon's glare. Even with telescopes, the moon's glare hides fainter objects. So, when we ask for telescope time, we have to specify if we can do our science when the moon is bright.

When the moon is bright, astronomers tend to look at bright stars, because their light cuts through the moon's glare. More recently, astronomers also tend to use the full moon time to look at objects in infrared light. Although the moon is still very bright in the infrared, the sky itself is always glowing brightly in the infrared, and the moon's light doesn't add much to the overall glow. (There are also stories of infrared astronomers moving the telescope to their next target and finding the moon is in the way!) But those of us who just want to study faint stars or galaxies in visible light are out of luck near the full moon. So, even as the modern world steers further away from lunar calendars, observational astronomers still have to pay close attention to the phases of the moon.

Jump for joy, it's Leap Day!

Image Credit: NASA

Today is Leap Day, February 29. Every four years, February gets to last an extra day, and the year is 366 days long. (Does this mean that places that say they are open 24/7/365 get to close for the day?) But why do we even have leap years?

First, we need to define what we mean by a "year." Forget the calendar for a minute. Why would we even want to define a year? Well, the year is one of the most fundamental cycles in our lives here on Earth. The seasons run through one full cycle in a year. The constellations appear to make one full circuit through the sky. The days get longer and shorter. And then it all repeats again.

This year, also known as the "tropical year," (yes, there are other kinds, but that's not really important for most people), is the amount of time it takes the sun to complete one full circle in the sky, due to the Earth finishing one full orbit around the sun. But the Earth doesn't take an exact number of days to finish its orbit. The tropical year is 365.24219 days long. So, in a "normal" year of 365 days, the Earth needs an extra 0.24219 days (about 1/4 day) to finish its orbit. Over four years, this almost adds up to one complete day. So, in order to keep the sun aligned with our calendar, we add an extra day to the year.

But if you do a little math, you find that the calendar still doesn't quite work. Three years of 365 days and one year of 366 days averages out to 365.25 days per year -- a little too much. Over a century, this cycle adds up to almost one full day. So, every century, we skip one leap year. 1896 was a leap year, 1904 was a leap year, but 1900 was not a leap year.

But even this scheme doesn't work out. Over a century, the average length of the year is 365.24 days, or 0.00219 days shorter than the real year. Over 400 years, we've lost almost one full day! So, every 400 years, we keep the leap day that would have been thrown out. That's why the year 2000 was a leap year, even though 1700, 1800 and 1900 were not leap years, and even though 2100, 2200 and 2300 won't be leap years. (At least they shouldn't be; I don't expect to be around to find out if we stick to our calendar or not.)

Over 400 years, the average length of our calendar year works out to 365.2425 days. That's an accuracy of one day every 3200 years. That's not too bad for a calendar that was developed 445 years ago! The astronomer John Herschel pointed out that we should consider not having a leap year in the year 4000, but most people aren't too worried about that quite yet!

Update: After posting this, I noticed that yesterday the Bad Astronomer posted a very similar blog entry, although he goes through the math in a bit more detail and with a bit more clarity. I guess Leap Day makes an easy target for us bloggers!

Blue moon, you saw me standing alone...

Image credit:Kostian Iftica, ExploreTheCosmos.com Today, at 10:04pm EDT, the second full moon of the month of May (as seen from the U.S.) will occur. In popular astronomy, this event (the second full moon in a single month) is called a "blue moon." The moon isn't really blue tonight, it's just a name given to the moon.

But even this name is not technically correct. First, astronomers use Universal Time (Greenwich Mean Time, or the time in Greenwich, England when it is not Daylight Savings Time). And, by that measure, the full moon occurs early in the morning on June 1 (1:04am UT, to be precise). So the astronomical blue moon will be next month, when the second full moon will be June 30 at 1:34pm UT.

Second, using the term "Blue Moon" to refer to the second full moon in a month started as a mistake. This article from Sky & Telescope magazine explains how that mistake was made. In the past, the term "blue moon" referred to the third full moon in a season that had four full moons instead of the normal three. But the mistake is now ingrained in popular culture, and there is no need to try and correct it (in my humble opinion).

Scientifically, there is nothing interesting about a blue moon. It takes the moon 29.5 days to go through a set of phases, and months (except for February) are longer than this. So, it is possible, with the exception of February, to have two full moons in a month. This happens every two and a half years or so, including this (or next) month.

In this way, a blue moon is like Leap Day -- it is an occasional event because our calendar (based on the Earth going exactly once around the sun) doesn't match up exactly with cycles of the moon's phases (a "moonth," if you like) or the Earth's spinning about its axis (a "day").

So, although there is nothing special about this full moon, I am happy for the press. Anything that can get people to go and look at the sky is a bonus. If you go out looking for the blue moon this evening, look for the planet Venus (by far the brightest "star" in the western sky after sunset right now.) The Big Dipper is high in the sky in the early evening, too. Just above and to the right of the moon is the star Antares, the heart of Scorpio. To the left of the moon, the bright "star" is the planet Jupiter. Compare the colors of Jupiter to Antares, and you might be surprised to notice that Antares is definitely reddish in color. (It may be a little hard because of the glare of the moon). And Venus is definitely whiter than either Jupiter or Antares!

Lunar Calendars

This week is a week of holy days for people of the Jewish and Christian faiths. For Judaism, the Passover celebration began last night; for Christians (both western and eastern), Easter is this Sunday. The exact days change from year to year. Why?

The Jewish calendar is based to a large extent on the moon. The moon completes a cycle of phases every 29.5 days; twelve "moonths" (or lunar months) add up to a bit over 354 days. Note that this is 11 days short of a normal, or "solar" year. So, the Passover celebration, which occurs in the Jewish religious month of Nissan, can shift with regard to the Gregorian calendar that we all use. Every few years, an extra month has to be added to the calendar to keep the lunar calendar in line with the sun, so that the Passover celebration always occurs in the spring.

The date of Easter used to be fixed to the date of Passover, since the Christian holiday is based on events that occurred during the Passover celebration. More recently, the date was changed so that Easter is the first Sunday following the first full moon of spring. In western Christianity, most years the two holidays coincide. In eastern (orthodox) Christianity, due to vagaries of different calendars used, the Easter celebration is often a lunar cycle later (though this year they coincide).

There are many other calendars based on the lunar cycle. The Islamic calendar sticks very precisely to the lunar cycle, so dates of Islamic holidays drift with respect to the seasons. Most other lunar calendars have an extra month added in now and then to try and keep track with the seasons.

Calendars are all, to some extent, artificial. There are not an even number of days in a lunar cycle (or in a year, which is why we need leap days now and then), there are not an equal number of lunar cycles in a year. But for the purposes of time keeping, a day, a lunar cycle, and a year are natural divisions of time if you don't have watches and calendars. In fact, in many ways the Gregorian calendar that we use is even more arbitrary -- a month doesn't follow the lunar cycle, and the lengths of the months vary from 28 to 31 days in an irregular fashion, and every 4th year (except every 100th year, except once again for every 400th year) we add an extra day to our calendar.

If you are celebrating holy days this week, a happy Passover or happy Easter to you all. And, even if you aren't celebrating, you know that the full moon is this week, so go out in the evening and enjoy the big yellow moon.