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No Perfect Calendar: Why We Have Leap Years and Why the Earth's Always out of Sync
By Zulfikar Abbany
Family lore has it my father was born on Feb. 29, 1936 — a leap day in a leap year many moons ago. It seems fitting for a man who says he's "not interested" in birthdays that he should have the option of missing his three years out of (almost) every four.
But given my dad's talent for spinning yarns (he started his working life in markets), it's possible he wasn't even born in 1936, but in 1935, and that was just a bog-standard, "common" year.
Hold On: What's the Difference Between a Common Year and a Leap Year?
In the Gregorian calendar, one of the most widely used calendars in the world, a common year is your standard "365-day" cycle. A leap year, however, has 366 days. It's an attempt at improving the accuracy of the Western-Christian calendar to keep it in sync with the Earth's rotations around the sun and "fixed" astronomical events, such as the equinoxes and solstices.
So What's the Problem?
Put simply: there's no perfect calendar. A calendar depicts a year, usually an imperfect year.
A year is the time it takes for the Earth to orbit the sun. We say it takes 365 days for the Earth to orbit the sun — but that's not strictly true. A true year — known as a tropical year, solar year, astronomical year or equinoctial year — is the time it takes the sun to pass from vernal (or spring) equinox to vernal equinox. That's 365 days, 5 hours, 48 minutes, and 46 seconds, or 365.2422 days to be precise. So there's roughly a six-hour margin of error in every "common year." Leap years compensate for the extra 0.2422 of a day. Failing to compensate for these "extra" hours would send us out of sync with the seasons — by about 24 days after only 100 years.
Just Explain That Again…
A common year is 365 days, which is about a quarter of a day shorter than a tropical year. At 366 days, leap years are three-quarters of a day longer than a tropical year. Over time, the combination of common and leap years keeps us roughly in sync with the Earth's orbit of the sun.
How Often Do We Have Leap Years?
Leap years occur almost every four years, with some exceptions.
What's This About "Almost" Every Four Years?
Leap years were introduced by that grand old Roman, Julius Caesar, in the Julian calendar. Back then, leap years occurred every four years without fail. But it was felt this over-compensated. The Gregorian calendar was then devised by Aloysius Lilius, an Italian astronomer and philosopher, (and named after Pope Gregory XIII) to replace the Julian calendar, with stricter criteria for leap years.
The Gregorian calendar stipulates leap years can be evenly divided by four, and century years, those ending in "00," are leap years if they are divisible by 400, but not if they can be evenly divided by 100.
We have to skip some leap years to account for the fact that those extra, or decimal hours (the 5 hours, 48 minutes, and 46 seconds) in the tropical year are just short of a quarter day. So in a sense we're re-adjusting the re-adjustment, but we're still left with an imperfect sum.
Battle of the Calendars
The Gregorian calendar was first adopted in Italy, Poland, Portugal and Spain in 1582. It is regarded as one of the most accurate calendars in use today. But it maintains a margin of error of about 27 seconds per year — that's one day in every 3236 years. It's fourth in line for accuracy behind the Mayan calendar from about 2000 B.C.E. (margin of error: one day in every 6500 years), the Revised Julian Calendar from 1923 (margin of error: one day in every 31,250 years), and the Iranian Solar Hijri calendar from the 2nd millennium B.C.E. (margin of error: one day in 110,000 years). The Solar Hijri is said to achieve its high accuracy by using astronomical observations rather than mathematical ones.
Do Other Calendars Have Leap Years?
Yes. The Chinese calendar has leap years featuring leap months — rather than leap days as in the Gregorian. A Hindu leap year also features an extra month. The Ethiopian calendar has 13 months, where the 13th month has five days in a common year and six in a leap year. The Islamic leap year occurs 11 times in a 30-year cycle. And a Jewish leap year has between 383 and 385 days, occurring seven times in a 19-year cycle.
And Does All This Leaping Do Us Any Good?
We do a fair bit of leaping about — even adjusting Universal Time with leap seconds to account for irregular changes in the Earth's rotation. It can be very important for people to feel they are in sync with time and astronomical events, for instance, for religious reasons, such as Easter, which is tied to the Spring Equinox. But if not for religious or, say, environmental reasons, does it really matter if the seasons drift from month to month, or if we lose hours and days over thousands of years? Would we even notice?
Reposted with permission from Deutsche Welle.
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By D. André Green II
One of nature's epic events is underway: Monarch butterflies' fall migration. Departing from all across the United States and Canada, the butterflies travel up to 2,500 miles to cluster at the same locations in Mexico or along the Pacific Coast where their great-grandparents spent the previous winter.
Millions of People Care About Monarchs<p>I will never forget the sights and sounds the first time I visited monarchs' overwintering sites in Mexico. Our guide pointed in the distance to what looked like hanging branches covered with dead leaves. But then I saw the leaves flash orange every so often, revealing what were actually thousands of tightly packed butterflies. The monarchs made their most striking sounds in the Sun, when they burst from the trees in massive fluttering plumes or landed on the ground in the tussle of mating.</p><p>Decades of educational outreach by teachers, researchers and hobbyists has cultivated a generation of monarch admirers who want to help preserve this phenomenon. This global network has helped restore not only monarchs' summer breeding habitat by planting milkweed, but also general pollinator habitat by planting nectaring flowers across North America.</p><p>Scientists have calculated that restoring the monarch population to a stable level of about 120 million butterflies will require <a href="https://doi.org/10.1111/icad.12198" target="_blank">planting 1.6 billion new milkweed stems</a>. And they need them fast. This is too large a target to achieve through grassroots efforts alone. A <a href="https://www.fws.gov/savethemonarch/CCAA.html" target="_blank" rel="noopener noreferrer">new plan</a>, announced in the spring of 2020, is designed to help fill the gap.</p>
Pros and Cons of Regulation<p>The top-down strategy for saving monarchs gained energy in 2014, when the U.S. Fish and Wildlife Service <a href="https://www.fws.gov/southeast/pdf/petition/monarch.pdf" target="_blank">proposed</a> listing them as threatened under the Endangered Species Act. A decision is expected in December 2020.</p><p>Listing a species as endangered or threatened <a href="https://www.fws.gov/endangered/esa-library/pdf/listing.pdf" target="_blank">triggers restrictions</a> on "taking" (hunting, collecting or killing), transporting or selling it, and on activities that negatively affect its habitat. Listing monarchs would impose restrictions on landowners in areas where monarchs are found, over vast swaths of land in the U.S.</p><p>In my opinion, this is not a reason to avoid a listing. However, a "threatened" listing might inadvertently threaten one of the best conservation tools that we have: public education.</p><p>It would severely restrict common practices, such as rearing monarchs in classrooms and back yards, as well as scientific research. Anyone who wants to take monarchs and milkweed for these purposes would have to apply for special permits. But these efforts have had a multigenerational educational impact, and they should be protected. Few public campaigns have been more successful at raising awareness of conservation issues.</p>
<span style="display:block;position:relative;padding-top:56.25%;" class="rm-shortcode" data-rm-shortcode-id="91165203d4ec0efc30e4632a00fdf57d"><iframe lazy-loadable="true" src="https://www.youtube.com/embed/KilPRvjbMrA?rel=0" width="100%" height="auto" frameborder="0" scrolling="no" style="position:absolute;top:0;left:0;width:100%;height:100%;"></iframe></span>
The Rescue Attempt<p>To preempt the need for this kind of regulation, the U.S. Fish and Wildlife Service approved a <a href="https://www.fws.gov/savethemonarch/pdfs/Monarch%20CCAA-CCA%20Public%20Comment%20Documents/Monarch-Nationwide_CCAA-CCA_Draft.pdf" target="_blank">Nationwide Candidate Conservation Agreement for Monarch Butterflies</a>. Under this plan, "rights-of-way" landowners – energy and transportation companies and private owners – commit to restoring and creating millions of acres of pollinator habitat that have been decimated by land development and herbicide use in the past half-century.</p><p>The agreement was spearheaded by the <a href="http://rightofway.erc.uic.edu/" target="_blank">Rights-of-Way Habitat Working Group</a>, a collaboration between the University of Illinois Chicago's <a href="https://erc.uic.edu/" target="_blank" rel="noopener noreferrer">Energy Resources Center</a>, the Fish and Wildlife Service and over 40 organizations from the energy and transportation sectors. These sectors control "rights-of-way" corridors such as lands near power lines, oil pipelines, railroad tracks and interstates, all valuable to monarch habitat restoration.</p><p>Under the plan, partners voluntarily agree to commit a percentage of their land to host protected monarch habitat. In exchange, general operations on their land that might directly harm monarchs or destroy milkweed will not be subject to the enhanced regulation of the Endangered Species Act – protection that would last for 25 years if monarchs are listed as threatened. The agreement is expected to create up to 2.3 million acres of new protected habitat, which ideally would avoid the need for a "threatened" listing.</p>
A Model for Collaboration<p>This agreement could be one of the few specific interventions that is big enough to allow researchers to quantify its impact on the size of the monarch population. Even if the agreement produces only 20% of its 2.3 million acre goal, this would still yield nearly half a million acres of new protected habitat. This would provide a powerful test of the role of declining breeding and nectaring habitat compared to other challenges to monarchs, such as climate change or pollution.</p><p>Scientists hope that data from this agreement will be made publicly available, like projects in the <a href="https://www.fws.gov/savethemonarch/MCD.html" target="_blank">Monarch Conservation Database</a>, which has tracked smaller on-the-ground conservation efforts since 2014. With this information we can continue to develop powerful new models with better accuracy for determining how different habitat factors, such as the number of milkweed stems or nectaring flowers on a landscape scale, affect the monarch population.</p><p>North America's monarch butterfly migration is one of the most awe-inspiring feats in the natural world. If this rescue plan succeeds, it could become a model for bridging different interests to achieve a common conservation goal.</p>
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