Leap years: proof that Earth is always running late

Unless you’ve been living under a rock, you’re probably aware 2016 is a leap year. February will have 29 days as part of a four-year mathematical cycle that has been observed in the Gregorian calendar since 1582. The reason we do this? To make up for a slight discrepancy in the Earth’s orbit around the sun.

As humans, we like to count things and measure our lives by predictable cycles, so Western cultures designed a calendar counting 365 days in a year. However, Earth’s true orbit is actually 365 days, five hours, 49 minutes and 16 seconds. That’s right; every New Year’s Eve, Earth is little under six hours late to the party. Talk about procrastination!

To make up for Earth’s tardiness, we add those six hours together every four years to make a full day. This keeps the calendar from drifting through the seasons over time. It might take a while, but if we didn’t add leap days, in 31 leap years (or 124 years), Jan. 1 would occur the first day in February. That means the Spring Equinox would happen Feb. 20 instead of in March!

But that’s not the end of the problem. In adding a day every four years, we overcompensate by 10 minutes and 44 seconds. (Remember Earth doesn’t really take another full six hours to complete its trip around the sun.) However, the Gregorian calendar accounts for this, as well.

A view of Earth by NASA.

Over a period of 400 years, the true length of the leap cycle, this overcompensation amounts to a total of three days. So in every century that isn’t divisible by 400, we don’t add a leap day. The last one was back in 1900. The year 2000, divided by 400, equals five, so we did observe leap year the February after Y2K. But you’ll have to live until 2100 to notice the next time we skip it. Your kids and grandkids will probably still be around, though, likely talking about lazy Earth and the crazy math behind leap years!

When the renovated Burke Baker Planetarium opens March 11, you can see leap years in action with a full map of Earth’s orbit, as well as the rest of the planets in the Solar System. Speed up time to compare rates and see how Earth measures up. Travel to the edge of our neighborhood and meet up with Pluto and other dwarf planets, and see how astronomers found evidence of a new Planet Nine! (I wonder how long that calendar is…)

Seeing Stars with James Wooten: March 2013

Jupiter is almost overhead at dusk, but now a little toward the west. Face high in the west at dusk and look for the brightest thing there (unless the Moon is also there), as Jupiter outshines all stars we ever see at night.

Saturn remains in the morning sky this month.  Look for it in the south/southwest at dawn.

Venus and Mars are on the far side of the Sun and out of sight this month. Venus passes behind the Sun (at superior conjunction) on March 28.

Brilliant winter stars shift toward the southwest during March. Dazzling Orion is almost due south at dusk. His three-starred belt is halfway between reddish Betelgeuse and bluish Rigel. Orion’s belt points up to Aldebaran in Taurus the Bull. This winter and spring the Bull also contains Jupiter.

To Orion’s upper left are the twin stars Castor and Pollux, marking the heads of Gemini, the Twins. You can find Sirius, the brightest star we ever see at night, by drawing a line from Orion’s belt towards the horizon. To Orion’s left, about level with Betelgeuse, is Procyon, the Little Dog Star.

From Sirius, look a little bit to the right and then straight down to the horizon. If your southern horizon is clear of clouds and tall earthly obstacles, you’ll see Canopus, the second-brightest star ever visible at night. This star is so far south that most Americans never see it and many star maps made in the USA omit it. (You must be south of 37 degrees north — the latitude of the USA’s Four Corners — for Canopus to rise). As you view Canopus, keep in mind that the sky we see depends on our latitude as well as on time of year and time of night.

Joining the winter stars are stars of spring rising in the east.  Look for Leo, the Lion at dusk. Later in the evening, extend the Big Dipper’s handle to ‘Arc to Arcturus’ and then ‘speed on to Spica’; these stars rise at about 10 p.m. in early March but by 9 p.m. on the 31st.

March 2013 evening skies feature an extra special object — comet Pan-STARRS, our first naked-eye comet since Hale-Bopp back in 1997!  Observers south of the equator have already been observing Pan-STARRS, but the comet has been invisible to us because it has been south of the Sun in our sky. That is beginning to change as Pan-STARRS nears its closest approach to the Sun late on March 9.

Like all comets, Pan-STARRS will be at its brightest as it comes closest to the Sun.  At the same time, Pan-STARRS will be coming up through the plane where the planets orbit and thus will be much easier for us to see in mid-March. You can start looking in western twilight as early as March 7 if you have a low, unobstructed horizon. The comet may be slightly easier to see on March 12 and 13, when the crescent Moon is nearby. Once Pan-STARRS appears in the western dusk sky, it shifts towards the north (to the right as you face west) each night, until it fades and returns to the Sun’s glare in April.

As always, scientists are unsure how bright Pan-STARRS will get.  It now seems that it won’t be as spectacular as was Hale-Bopp in 1997. However, southern observers are seeing it naked-eye, and so should we. The comet could be about as bright as average stars such as those in the Big Dipper, but may be dim enough that you need a dark site to see it, especially once the Moon gets bigger.

Ultimately, though, we’ll have to wait and see. Sky and Telescope has a helpful finder chart here.

Moon Phases in March 2013:
Last Quarter                  March 4, 3:54 pm
New                               March 11, 2:53 pm
1st Quarter                    March 19, 12:26 pm
Full                                March 27, 4:29 am

At 6:01 a.m. on Wednesday, March 20, the Sun is directly overhead at the equator.  This therefore is the vernal (spring) equinox, a day when everyone on Earth has the same amount of daylight. After this date, our gradually lengthening days become longer than our nights, and we go into springtime. South of the equator, days have been shortening.  For them, this equinox marks the point when night becomes longer than day, and the onset of autumn.

Sunday, March 10, is the second Sunday of this month. Therefore, Daylight Saving Time begins at 2 a.m. this morning (the time officially goes from 1:59 to 3 a.m.)  Don’t forget to spring forward by advancing all clocks one hour on Saturday night, March 9!

On most clear Saturday nights at the George Observatory, you can hear me do live star tours on the observation deck with a green laser pointer.  If you’re there, listen for my announcement. This Spring Break, the George will be open to the public on Tuesday night, March 12, and Thursday night, March 14.

To enjoy the stars in any weather from the comfort of the HMNS Planetarium, click here for a full schedule.

Go Stargazing! March Edition

Venus leaves the evening sky in dramatic fashion this month.  Look west-southwest right as night falls for the brightest thing there except for the Moon.  Keep watching each clear night this month; you’ll see Venus noticeably lower to the horizon each passing day.  By March 20, Venus sets as twilight ends, and by the end of the month, it is gone.  When is the last day the month you can see it?  Venus, on its faster, inner orbit, has come around to our side of the Sun and will pass us on March 27.  Astronomers call this alignment inferior conjunction

 photo credit: rich115

In addition, Venus’ orbit is highly inclined to ours.  (The planets orbit almost, but not exactly, in the same plane.)  As a result, we often see Venus pass above or below the Sun at inferior conjunction rather that truly aligning with the Sun.  This time, Venus passes ‘above’ the Sun in our sky, giving us the chance to see it as both evening and morning star!  Do you have a clear horizon, without tall trees or buildings, to the east and west?  If so, then you can try observing Venus very low in the west at sunset and very low in the east the next morning.  It’s best to try this between March 24 and 27.

Saturn is now up all night.  On March 8, Earth passes between the Sun and Saturn, putting the Sun and Saturn on opposite sides of the Earth.  In this alignment, called opposition, a planet rises at sundown and sets at sunup; it is visible literally all night long.  Saturn is nowhere near as bright as Venus, but it is in a relatively dim star field and therefore is just as easy to see.  Face east at dusk, south at midnight, or west at dawn to see it.

Mars and Jupiter emerge from the Sun’s glare this month.  Jupiter, in the southeast at dawn, is the brightest thing in that part of the sky unless the Moon is nearby (as it is on March 22, 23, and 24).  Mars moves faster than Jupiter and therefore seems to ‘keep pace’ with the Sun’s apparent motion.  As a result, Mars remains close to the horizon at dawn much of the spring, and takes longer to fully emerge into the morning sky.

 photo credit: makelessnoise

Dazzling Orion is due south at dusk.  His belt points up to Aldebaran, the eye of Taurus, the Bull.  The Dog Stars Sirius and Procyon are below Orion in the east.  Sirius is the brightest star we ever see at night.  Look for a fairly bright star just to the right of Sirius and then drop your gaze straight down to the horizon. The bright star just above the horizon, possibly shining through trees, is Canopus, the second brightest star we see at night.  This star is so far south that it never rises for people north of 37 degrees north latitude (Houston is at just under 30 degrees north).  To Orion’s upper left are two stars of similar brightness less than five degrees apart.  These are Castor and Pollux, marking the heads of Gemini, the Twins.  Look in the east at dusk for stars in the shape of a backwards question mark, with a right triangle below that.  These stars are in Leo, the Lion.  Saturn rises in Leo.

Moon Phases in March 2009:

1st Quarter           March 4, 1:45 am
Full                       March 10, 9:37 pm
Last Quarter          March 18, 12:49 pm
New                      March 26, 11:07 pm

 photo credit: bogenfreund

Sunday, March 8, is the second Sunday of March.  Therefore, we spring forward to Daylight Saving Time at 2 a.m. that morning.  (Clocks officially go from 1:59 a.m. to 3 a.m.)  Don’t forget to set your clocks one hour ahead Saturday night, March 7!

At 6:45 am on Friday, March 20, the Sun is directly overhead at the equator.  This is therefore the vernal (spring) equinox.  On this date, everyone has the same amount of daylight.  For us, day is now longer than night, and days will continue to lengthen until June.  In the Southern Hemisphere, it’s night that is getting longer.  For them, this is the autumnal equinox—the start of fall.