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February's Last-Quarter Moon: Why It's Missing

The reason is the sun, moon and Earth don’t move to the tune of simple arithmetic.

As you gaze at the first-quarter moon this week, you may wonder when the last-quarter moon will occur this month. But there won't be one, if you live in North or South America.

Take the situation in eastern North America, which is in the Eastern Standard Time (EST) zone. The previous last-quarter moon was on Jan. 31 at 10:28 p.m. EST, and the next one will be on March 1 at 6:11 p.m.

The mathematics behind this is that the average synodic lunar month - from new moon to new moon - is 29.53 days long, while February is either 28 or 29 days long. So it is possible, even in a leap year like 2016, to have one of the four main lunar phases fall outside the calendar month of February. [Earth's Moon Phases, Monthly Lunar Cycles (Infographic)]

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Other parts of the world, such as Europe, had a last-quarter moon this month early on the morning of Feb. 1.

We make a big fuss about the "Blue Moon," when there are two full moons in a month, but we don't seem to notice when one of the lunar phases goes missing.

This raises the question of why our months vary so much in their number of days: 28 or 29 in February, 30 in April, June, September and November, and 31 in the other seven months. The problem is that the sun, moon and Earth don't move to the tune of simple arithmetic.

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The lunar month consists of 29.530589 days, and the tropical year (equinox to equinox) is 365.242190 days long. When the ancient astronomers attempted to construct a calendar with these bizarre numbers, they found that, literally, it did not compute.

Early astronomers divided the shape of a circle into 360 degrees. This seems like a strange number to us with our decimal system, but it made sense with a number system based on 12. It also came close to the number of days in a year, though not close enough. The year was divided into 12 months (a natural in a base-12 number system) of 30 days each - but that left the awkward 5-and-a-bit-days remaining.

Mathematicians struggled with this problem for thousands of years, until finally a papal commission in 1582 came up with a complex but elegant solution, known as the Gregorian calendar, after Pope Gregory XIII, who commissioned it.

In order to get the church's feast days back in phase with the astronomical calendar, it was necessary to omit 11 days.

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The Pope had the power to enforce the new calendar in Catholic countries, though there was a bit of grumbling about the 11 days, which went missing between Oct. 4 and Oct. 15, 1582. Just for fun, try entering Oct. 4, 1582, in a planetarium program like Starry Night, and then advance to the next day. You will find it is Oct. 15.

Naturally, England (along with its North American colonies) was one of the strongholds of the old Julian calendar, and resisted adopting the popish Gregorian calendar until 1752. By that time, the calendar used by the English was off by 12 days, so that Sept. 2, 1752, was followed by Sept. 14, 1752, in England and its colonies.

To avoid the missing-days problem, we now have a system of 30- and 31-day months, with poor February being stuck with making the whole thing fit. Thus, we have 29 days in February every four years, with a few exceptions to fine-tune the length of the year over the centuries. The result is that most months are a little longer than the lunar month, so they sometimes have two full moons (or other double phases). And February sometimes ends up missing a phase, as happens this year.

This article was provided to Space.com by Simulation Curriculum, the leader in space science curriculum solutions and the makers of Starry Night and SkySafari. Original article on Space.com.

Copyright 2016 SPACE.com, a Purch company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

A quarter moon, shown here, will be missing at the end of this month.

Apollo has never looked better. More than 40 years after the astronauts explored the surface, two photo projects are showing us the moon as only a handful of people has seen before.

The Project Apollo Archive

uploaded thousands of scanned NASA images to Flickr. Also, a new crowdfunded book called "

Apollo: The Panoramas

" easily exceeded its initial goal and will begin shipping hardcover books next year. This is some of the science these photos helped NASA perform.

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Some rumors circulated in the late 1960s that the first Apollo astronauts to land on the moon may very well sink into the surface. That wasn't really the worry for NASA during Apollo 11; after all, the Surveyor 3 (U.S.) and Luna 9 (Soviet) spacecraft had arrived safely, among others, with little evidence of subsidence. But NASA was interested in how well the lunar module performed when astronauts arrived on the surface. In pictures, NASA surveyed information such as how far the feet penetrated into the surface, and how much of a divot the engine exhaust left behind.

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NASA was also interested in knowing how the investigators' experiments worked on the surface. Some of the experiments were used multiple times in missions, such as the foil solar wind composition collector seen here from Apollo 12. By asking the astronauts and looking at pictures of the deployed experiments, the investigators could make improvements from mission to mission to better data collection or other aspects of the mission.

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Without the lunar rover, the Apollo missions would have had severely limited surface operations. The rovers could carry equipment, samples and astronauts for many miles across the surface, allowing for more intensive investigations. NASA took care to make sure the astronauts observed a "walkback limit", just in case the rover broke down and the crew had to hike back to the lunar lander.

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As the Apollo program matured, the astronauts received advanced training in geology so they could better make choices about their work on the surface. This allowed them to select rocks representative of the environment, and to give detailed descriptions to Mission Control about their surroundings that could be recorded for the geology team. As a part of that, Apollo 15 commander Dave Scott did a brief survey of the landscape before even setting foot, perched inside the lunar lander and sticking his torso outside to take pictures and relay information to NASA.

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Where were the astronauts on the surface of the moon? That was no trivial question after astronauts began using rovers to get miles away from their landing site -- this shot from Apollo 16 gives you a taste. NASA closely monitored the astronauts' discussions, looking at television cameras and trying to plot their best estimates of the crew's location on orbital maps. Later on, when the pictures were developed, NASA and others looked at them to pin down where the astronauts physically were.

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While the astronauts described their surroundings as best as possible, NASA had a special color tool available to "calibrate" the images on the moon to their true color. An identical copy of this scale was on Earth, making it possible to do comparisons from afar as to what color the moon's regolith (soil) really was. That turned out to be very important on Apollo 17, when the astronauts found what they thought was orange regolith in an otherwise greytone landscape. They were right; it was the tint of volcanic glass.

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