23 January 2017

Solar Eclipses and the Saros Cycle

Moon's Ascending and Descending Nodes
Eclipses are the outcome of a chance series of alignments between three bodies: the Sun, Moon and Earth. Orbital mechanics and the laws of Kepler ensure that these bodies circle each other in a beautiful series of harmonious ellipses, near perfect circles each with their own periodicity and in the case of the Moon, with its own orbital inclination. The interplay between the various cycles of the Moon orbiting the Earth, the Earth orbiting the Sun, and the Moon’s gradually changing orbital inclination lead to patterns that repeat over short, medium and long periods of time as these three bodies align.
Eclipses of Saros 145

One of the overall epicycles of these orbits is called the Saros Cycle. At any given time there are many Saros Cycles occurring coincidentally and the Great Eclipse on August 21st is a member of Saros 145. What does this mean? A Saros is an 18 year cycle in which three of the Moon-Earth cycles repeat nearly perfectly, the end effect of which is to create a near duplicate eclipse in this long period of time. The eclipse across Europe in August 1999 was a member of Saros 145, the most recent one of that Saros series until this coming August. In the intervening 6585 days there have been many other lunar and solar eclipses, but none with the exact geometry and timing that we saw in August 1999. So if you get a chance to see the eclipse in August 2017, know that it is virtually identical to the one experienced by observers in Europe 18 years ago. I was in Hungary for the 1999 eclipse and look forward to seeing the Great American Eclipse this summer, a chance to see an old friend again, a pleasant 2 minutes and 40 seconds where I am aligned with the Sun, Moon and Earth but not only just aligned, but in the specific geometrical arrangement that I witnessed 18 years prior. 

For a somewhat deeper look into the Saros Cycle, here are the basic three motions. (1) The Moon’s orbit around the Earth combined with the Earth’s orbit around the Sun leads to the well-known phases of the Moon that repeat every 29.5 days, the time from New Moon to New Moon, and we call that the Synodic Month. (2) At the same time, the Moon’s elliptical orbit around the Earth means we have a close approach and a more distant approach every month and the time between two successive closest approaches (Perigee) is 27.55 days, a time period that is called the Anomalistic (Perigee) Month. (3) Finally the Moon orbits the Earth on a slightly inclined orbit so at times the Moon is ascending from below to above the plane of the solar system (‘ascending node’) or descending from above to below the ecliptic plane (‘descending node’). The time period from one ascending node to the next is called a Draconic Month which is 27.21 days. If you combine all three of these time periods, they nearly perfectly repeat after 18 years and 10 or 11 days (depending on the number of Leap Years between the two years in question), constituting the repetition period of consecutive eclipses in a Saros Series. Saros 145 eclipses took place on August 11th 1999 and on August 21st 2017. And of course the next Saros 145 eclipse will arrive on September 2nd 2035, right on schedule!

For more on this subject, Wikipedia has an excellent write up

Images courtesy NASA. 

02 January 2017

Seeing the Andromeda Galaxy

Pegasus and Cassiopeia
While out under the night sky I frequently point out that everything we can see with the naked eye is located in our home galaxy, the Milky Way. The thousands of stars that shine in a dark night sky are, relatively speaking, local stars in our own galaxy. The Milky Way is vast, stretching 100,000 light years from end to end. To see anything beyond our own galaxy means we are seeing well past 100,000 light years.

The Andromeda Galaxy is a neighboring galaxy in our 'Local Group' and is the nearest fully-formed galaxy. Despite its size (about 50% larger than our own Milky Way galaxy) and overall brightness, it is located 2.2 million light years away so it is an object that only under very good conditions can be glimpsed by the naked eye, but even then is challenging to spot and is best viewed with some magnification. My preference is to find Andromeda with binoculars and in the winter it is a good target because it is located directly overhead. With warm clothes and a comfortable blanket or pad, you can lie on your pack and look up with binoculars, and with some attention and focus you can see beyond our galaxy into Andromeda.

The Andromeda Galaxy
My way to find the galaxy is to look between on the Great Square of Pegasus and Cassiopeia, finding the galaxy in the space between the two. The first image (above) shows the overall proximity of the Andromeda constellation between Pegasus and Cassiopeia. The second image (right) shows a more close up view for pointing your binoculars. As you search this part of the sky under reasonably dark conditions you will be able to see the glow of Andromeda come into view in your binoculars.

Images courtesy of Sky Safari.

29 December 2016

Quadrantid Meteor Shower 2017

The first meteor shower of the year, the Quadrantids, peak on the morning of Tuesday January 3rd for those of us on the west coast of the United States. The Quadrantids have a relatively short peak (a few hours) and if you have favorable conditions with a dark cloudless sky, it will be worth the effort to brave the cold weather and look for these meteors.

Here are some other resources on the subject. The Sky & Telescope article provides a good history of this particular shower and both have tips on when and where to look.

Earth Sky: http://earthsky.org/?p=4287

Sky & Telescope: http://www.skyandtelescope.com/astronomy-news/catch-the-quadrantid-meteors-if-you-can/

14 December 2016

Winter Astronomy Highlights

Winter is nearly upon us, and the magical sky that arrives at the end of each year lights up the long dark night with many colorful stars, bright constellations, and treasures that await your viewing. When the sky is crisp and clear, put on a warm coat and grab a pair of binoculars and face south and you will find a sky full of ... more than just stars.

Universe2Go created a nice infographic that includes a timeline of specific events of interest over the next three months. Next up is a close alignment of the waning Moon and Jupiter on December 22nd, one day after the Winter Solstice. Mark your calendar for those and other astronomical events of interest.

Image courtesy of Universe2Go.

24 November 2016

Geminid Meteor Shower 2016

Geminid Radiant in Gemini
Every year there are many meteor showers that arrive on schedule, resulting from the fact that Earth travels through well-established regions of dust and particles on its annual trek around the Sun. The Geminids are one of the year's best, reliably peaking as the Earth passes through the remants of asteroid 3200 Phaethon on December 13th and 14th each year.

One of the most important factors in viewing a meteor shower is to find a dark sky and this year, unforunately, this will be very difficult due to the fact that the full Moon coincides with the peak of the Geminids. So although many meteors will streak into the upper atmosphere and burn up, most won't be visible because their light will be drowned out by the glowing moonlight in our atmosphere.

That being said, if the weather is clear and you have warm clothing, it will be a fine night to just sit out in your garden or a park or on a mountainside and look anyhow. The Geminids reach their peak much earlier than most meteor showers, so a pleasant hour of viewing in the evening should reveal a few of the brighter meteors. Just temper your expectations and you will have a nice night. You won't see hundreds of meteors per hour, but then again, there will be plenty of stars and if you have binoculars, you can turn it into an evening of reacquaining yourself with the brighter stars and nebulae of the winter sky as Orion climbs high into the eastern sky after sunset and Pegasus dominates the sky overhead.

Here are excellent online resources for further reading on meteor showers:

Sky & Telescope

NASA

EarthSky

Image courtesy of Sky & Telescope.

11 November 2016

Supermoon on November 13 and 14

How much bigger is a Supermoon? 
The natural cycles of the cosmos brings us many interesting sights in the night sky and our closest celestial neighbor, the Moon, is always one of the best and brightest as it aligns with the Sun for an eclipse, or a planet for a spectacular conjunction. Sunday and Monday November 13 and 14, the Moon reaches its closest point on its orbit around the Earth at nearly the exact same time as it reaches its full phase, creating a Supermoon. This special situation happens every year or so, but this time the Moon will be a slight bit closer to the Earth, making this the closest Supermoon since 1948. The next Supermoon of this stature will arrive in 2034. So if you have clear skies Sunday or Monday, make a special effort to get out and see this ever-so-large object in the sky, 14% larger than it is when the Moon is at its farthest from the Earth.

The exact moment of the full Moon is Monday morning at 5:52 am pacific time, and the closest approach to the Earth (perigee) at 3:22 am, a bit earlier that night. So if you want to see the biggest and best view that will be early Monday morning November 14th. However, moonrise on both the evening of the 13th and 14th should be incredible so don't feel obligated to see it at the exact moment if you prefer sleeping in a warm bed! Or if you happen to live in Europe you can simply watch the moonrise on the evening of Monday 14th and you'll be all set. The exact time of moonrise can be found here, depending upon your location.

Here are some good online resources to learn more: 



Image courtesy of Wikipedia

10 November 2016

Three Planets in the Sunset Sky

Three planets are visible in the west just after sunset. Saturn is on a gradual fade into twilight and will disappear from view later this month, following the path of the zodiac constellation Sagittarius and the center of the Milky Way as the Earth's steady motion around the Sun carries us from the summer and fall constellations into the winter skies.

Mars, Venus, Saturn
Venus is the brightest object in the west for the coming months, maintaining its position as a bright beacon above the horizon for an hour or two after sunset. Both Venus and Mars are not 'pulled' into the western horizon in the same way as Saturn because they are much closer to the Earth and more nimble, moving steadily east in their orbits and keeping pace with the Earth's own motion. For that reason they remain in the evening sky after sunset with Venus eventually dropping out of sight late this winter and Mars holding its western location until nearly summer.

To see any of this you will need a clear and unobstructed western horizon. A pair of binoculars will enhance the view. And clear skies!

07 August 2016

Perseid Meteor Shower 2016

One of the finest meteor showers of the year, the Perseid Meteor Shower, arrives over a two day viewing window from 11 to 12 August. With a first quarter moon setting near or shortly after midnight, the Moon will provide some distraction during the evenings, but the dedicated meteor seeker will be rewarded with dark skies after midnight when meteor showers naturally reach their peak.

To see the Perseids (or any meteor shower, for that matter), you will be best served by a dark location away from streetlights and ideally away from city lights. If you are in a fully light polluted city and cannot get to a dark rooftop or garden, then your chances of seeing many meteors are poor. However, any step you can take to eliminate light will help, from switching off your own house lights to avoiding streetlights to traveling outside of a city to the beach, desert or mountains. Why? Meteors come in all shapes and sizes and during a meteor shower many of the meteors that sail into Earth's atmosphere produce only a brief, faint trail of heated gas in their wake. Some, of course, are large and visible for longer periods, cover a greater portion of the sky, and are so bright that you cannot miss them. So to improve your odds of seeing more meteors and increase your enjoyment, find a dark spot, bring a lawn chair or blankets so you have a good view of the sky overhead, and relax while the show unfolds. And be prepared to stay up late for the best viewing.

Meteors are also fickle in that they come in groups, sometimes with two or three brights ones in a minute, and then there might be 10 minutes in which you see none and you start to give up hope. But the Perseids will deliver so you just need to be patient.

Universe2go created a great infographic that I have attached to this blogpost. Click to enlarge or download and print. It's a good resource guide for the meteor shower.