A Leap Day

Today is February 29th, that odd date that only occurs every four years.

The reason for a leap day inserted into the calendar, the existence of February 29th, is ultimately astronomical. Perhaps a little explanation is in order…

We originally defined days as the time it takes the Earth to rotate. While we define years as the time it takes the Earth to orbit once around the Sun. The problem is that these values do not divide evenly into one another.

Mauna Kea Sunrise
Sunrise seen from the summit of Mauna Kea

The Earth takes about 365.24219 days to obit the Sun, when measured by the Sun’s position in the sky, what is called a tropical year. There are different ways to measure a year, but if one is concerned with keeping the seasons in sync with your calendar, then you are interested in tropical years.

It is that bunch of decimals, the 0.24219 etc., that is the problem, every four years the count drifts out of sync by roughly one day. The insertion of an extra day every four years helps bring the calendar back into synchronization with the orbit of the Earth and with the seasons.

Even leap years do not quite fix the problem as 0.24219 is close, but not quite 0.25 or one quarter of a day. Thus additional corrections are needed… Enter leap centuries.

Our current calendar was instituted by Pope Gregory XIII in 1582, setting up a standard set of corrections for the fractional difference between the length of a year and the length of a day. Scholars knew that errors had been accumulating in the calendar for centuries, resulting in a drift of several days.

Religious authorities were concerned that this drift had displaced important celebrations in the church calendar, in particular the celebration of Easter. After much argument it was decided to reform the calendar. The current solution was devised by a number of astronomers, including Aloysius Lilius, the primary author of the new system.

The Gregorian Calendar uses an extra day in February every four years, unless the year is divisible by 100, then there is no leap leap day that year. However, if the year is divisible by 400, then it is a leap year. While this may sound odd, it does create a correction much closer to the ideal value of 365.24219 days per year.

I am a geek, so let us put that into code…

Even this is not perfectly precise. The correction is close but will drift given enough time. The length of a tropical year also changes slowly over time. We will eventually have to add another correction to keep the calendar and the seasons in sync. But not for a few millennia, good enough, for now.

As 2020 is divisible by four and not divisible by 100, there will be a leap day added to the end of this February… Today.

Jupiter at Superior Conjunction

Jupiter 14Apr2016
Jupiter on April 15, 2016

Today Jupiter passes through superior conjunction, rounding the far side of the Sun as seen from our earthbound vantage point.

Jupiter will re-emerge in the dawn sky towards the end of the January. Look for the planet low in the glow of dawn, rising higher each day.

Jupiter will pass through opposition on July 13th, 2020, crossing into the evening sky.

Winter Solstice

The winter solstice will occur at 18:19HST today.

Sunrise over Georgia Strait
Sunrise over Georgia Strait

The Sun has reached the most southerly declination it will achieve this year. After today the Sun begins to move north in our skies.

Today will be the shortest day of the year for those of us located in the northern hemisphere.

Today is considered the start of winter for most cultures in the northern hemisphere, or the start of summer for those in the southern hemisphere.

2019 Apsides and Seasons
Event Universal TimeHawaii Standard Time
Perihelion Jan 0305:20UTJan 0219:20HST
Spring Equinox Mar 2021:58UTMar 2011:58HST
Summer SolsticeJun 2115:54UTJun 2105:54HST
Aphelion Jul 0422:11UTJul 0412:11HST
Fall Equinox Sep 2307:50UTSep 2221:50HST
Winter SolsticeDec 2204:19UTDec 2118:19HST
Data from US Naval Observatory Data Services

Mercury at Maximum Elongation

Mercury, Venus and Jupiter
An evening conjunction of Mercury, Venus and Jupiter on 30 May, 2013

Today Mercury is passing through maximum elongation, the furthest it will rise above the rising Sun in the dawn sky. After today the planet will slide back into the Sun’s glare headed for superior conjunction on January 10th.

This is a modest apparition, with the planet only 20° from the Sun.

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New Moon

Young Moon
A very young moon over Waikoloa

New Moon will occur today at 05:06HST.

Over the next few days a slim crescent Moon will slide through a number of bright planets low in the sunset including Jupiter, Venus, and Saturn.

This coming weekend, Nov. 30th, offers an excellent night for observing with a slim 4 day old Moon in the sunset. Get out and enjoy the dark!

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Venus and Jupiter

Observant sky watchers will have noticed that the two brightest planets in our skies have been drawing close together. Jupiter and Venus are currently about 2° apart. Sunday evening will see them at their closest for viewers in the islands at just under 1.5° apart. Monday will see the pair very slightly further apart at just over 1.5° separation.

Venus and Jupiter over the Keck 1 dome

After Monday the two will gradually separate further with Jupiter disappearing into the sunset in mid-December while Venus continues to climb further from the Sun until maximum elongation on March 24, 2020.

On Wednesday the 27th a thin crescent Moon will join these bright planets, just 5° below Jupiter. On Thursday the 28th the Moon will be 4° above Venus. The three should make for quite a spectacular sight in the glow of sunset.

A degree and a half separation will allow both to fit in the field of view of very low power telescopes and binoculars, a bit much to fit both in the field of view of most telescopes.

Mercury Transit 2019

It was a clear predawn sky that greeted Waimea this morning, perfect to watch the transit of Mercury across the Sun.

The transit of Mercury on the morning of Nov 11, 2019
The transit of Mercury on the morning of Nov 11, 2019

An alarm set for o-dark-thirty and a drive to Waimea with the first glow of dawn behind the mauna. I did not have to pack a ‘scope as I would be using an observatory outreach telescope, just make sure I have camera gear ready.

Realistically I was expecting only a few folks in addition to the club members I knew were coming. A light crowd maybe? Thus I was rather surprised to find the parking lot filling quickly and our big conference room buzzing at 6am.

It was quite the crowd considering the Sun had not yet appeared over the shoulder of the mauna!

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