15 December 2019

The Long Solstice Season Begins

Image courtesy calaware.org 
This time of year, as darkness sets in early and the cold of winter envelops the northern hemisphere, I savor the changes that make each day different from the previous one. The Winter Solstice takes place this year on December 22nd and represents the shortest day at all locations in the northern hemisphere. Literally, Solstice means that "the Sun stands still" and in the context of the Sun's annual meandering (from our point of view) higher or lower above the horizon at solar noon, the Winter Solstice is when the Sun ceases to drift southward as measured at mid-day and begins the six month journey north. In my view, however, the Solstice is not just that one day and moment in time but rather a season, approximately one month in duration, during which the changes to the time of sunrise and sunset as well as the total length of the day go through rather fascinating changes.

As the Earth orbits the Sun every 365 1/4 days, its orbital speed is higher in December and January. The orbit of the Earth is elliptical and that ellipse brings the Earth slightly closer to the Sun in December and January and the speed of our orbit around the Sun goes up just enough that it has a measurable influence on the length of the day. The Earth always takes the same amount of time to go around once on its axis (one day), but because we are moving more rapidly around the Sun as we are at the inside of our elliptical orbit, the time period from one local noon (exactly when the Sun is at its highest point of the day) to the next varies by a few minutes, plus or minus, as we travel through this faster period of time. The consequence of this changing speed of the Earth means that the earliest sunset of the year happens one to two weeks before the Winter Solstice, and the latest sunrise one to two weeks after. The total effect of the early sunset and late sunrise meet 'in the middle' and create the shortest day on December 22nd, so the Solstice is very much still the shortest day. But the two other effects are quite noticeable.

Depending on your latitude north of the equator, the exact date of the earliest sunset and latest sunrise vary a bit. Using the tables on the timeanddate.com website, I entered my location here in Munich Germany and can see that the earliest sunset (16:19 or 16:20) is happening from December 5th through the 18th. Skipping ahead to January, I can also see that the latest sunrise (8:04) happens from January 1st through the 3rd. The exact timing depends not only on your latitude north of the equator, but also your location in your timezone, and varies if you are more to the east or west of the center of your timezone. Plug in your city and see how things look, and you can start to get a sense of your own Solstice Season.

Due to the nuances of the Earth's elliptical orbit around the Sun, the same effect happens in June but is much less pronounced around the Summer Solstice. So this is the best time of year to be attentive to the subtle changes that are happening in our sky and on our Earth, and by the time is it early January you can already start to celebrate the very gradual return of light to the daily rhythms of the northern hemisphere.

If you want to learn more, here are two good articles on the subject, one from Bruce McClure at EarthSky.org, and another by Aparna Kher on TimeandDate.com.

10 December 2019

Geminid Meteor Shower 2019 & Full Moon

Geminids
Everyone loves a good meteor shower. And the Geminids, which peak mid-December, are one of the best each year. But all meteor showers are subject to the whims of our own Moon which can substantially disrupt viewing. The full Moon is a delight to look at, both naked eye and through a telescope. But it casts a significant glow in the night sky and that means that the fainter meteors in a meteor shower will be virtually impossible to see. With the Moon full or nearly full from December 11th to the 14th this year, it will be shining brightly exactly when the Geminids are expected to reach their peak on the 14th.

Nevertheless, I take encouragement from this excellent article from the American Meteor Society that  provides some good tips for seeing a few of the brighter Geminids this year. Good luck, dress warm, and clear skies!

Image courtesy of Wikimedia Commons / Asim Patel

25 November 2019

Planetary Alignments

Jupiter, Venus, Saturn and the Moon put on a good show this week in the evening sky. To see this you will need a very good western / southwestern horizon and clear skies. As Jupiter and Saturn recede into the sunset sky a bit lower each evening, Venus is racing out of the glow of the sunset sky and emerging as our evening 'star' for December and the coming months.

Of course, the planets and the Moon all occupy roughly the same path across the sky, so when planets align the Moon is often not far away. On Wednesday and Thursday and Friday the Moon will closely pass below and then above the Jupiter-Venus conjunction and then a day later, Saturn. The young Moon is always a treat to view in binoculars or a telescope, and having Venus and Jupiter and Saturn nearby will make the apparitions all the more exciting. The challenge here is that you will need to look very shortly after sunset, because within an hour most of these objects will set. Daily sky maps are here.

Clear skies!

Image courtesy of Sky & Telescope.



Follow Up Note: on November 28th I was on a plane flight above Germany and took this photo looking south west during the flight. The Moon is nicely placed between Venus and Jupiter. Venus is quite easy to find but Jupiter takes more work. Note that the Sky & Telescope diagrams (above) are based on the view from the US, so the Moon is past Venus, whereas I saw the alignment many hours earlier when the Moon was still between the two planets.


14 November 2019

Meteors, Meteors, Meteors!

November brings the annual Leonid meteor shower which peaks on Sunday November 17th. This year we have a second potential show, a "Meteor Outburst" on the evening of November 21-22. Unforunately the Moon will interfere quite a bit with the Leonids, but will have considerably less impact a few days later for the meteor outburst. Let's look at these two events in more detail.

Leonid Meteor Outburst in 1833
Meteor showers are some of the most exciting spectacles to watch in all of astronomy. However, the best views require dark, clear eastern skies and a willingness to be awake when most people are sleeping. And this time of year, they require warm clothes! If skies are clear, the Leonid Meteor Shower should be visible this weekend during the night of of November 17-18, but because of the 3/4 lit Moon, the sky will be glowing and only the brightest meteors will be visible.

Meteors are tiny bits of rock and dust that enter the earth's atmosphere and burn up. These bits of rock and dust are floating in long orbits in space and the earth "runs into" these clouds of rock and dust. Because the earth is moving so fast, the rocks and dust that are struck by the earth heat up from the friction of earth's atmosphere. The result of this are brilliant streams of light that are often called "shooting stars" but aren't stars at all, just very small visitors that shine briefly and flicker out.

The meteor shower coming up this weekend is called the Leonids and it is the result of the meteor stream from a comet known as Temple-Tuttle (comets are usually named after their discoverers). The shower is called Leonids because the comets appear to originate in the part of the sky where we find the constellation Leo the Lion. This constellation does not rise in the east until very late in the night and as such, we don't get the best view of the Leonids meteors until after midnight. This article by Deborah Byrd on EarthSky is very informative about the Leonids this year.

The "Meteor Outburst" a few days later is due to a similar cause as the Leonids themselves, the debris stream from a yet-to-be-discovered comet that has left a trail through which Earth will pass. That will happen on the night of November 21-22. This article from Sky & Telescope Magazine does an excellent job outlining the circumstances of the outburst and indicates that in the US you can expect a peak on the 21st around 8:50 pm pacific, 11:50 pm eastern, and here in Munich Germany around 5:50 am on the morning 22nd. I'll be up early to see if in fact we will get 100s of meteors storming in all at once. Can't wait!

Image credit: Public Domain / Wikimedia Commons

06 November 2019

Transit of Mercury on November 11, 2019 - don't miss it !

On Monday November 11 we will have a chance to witness a very unique and special event, a transit of the planet Mercury across the face of our Sun. This is a rare event, happening only 13 times in a century. The next one won't take place until 2032.

What is a transit? It is a precise alignment of the Sun, Earth and another celestial body. In this case, that body is Mercury, the closest planet to the Sun and a fairly small planet. It orbits the Sun every 88 days, and when we have this special alignment with a transit, we are witnessing the proper motion of the planet as it progresses in its orbit around the Sun. Mercury's diameter is 194 times smaller than the Sun so it will in fact appear as a tiny, perfectly round black dot against the surface of the Sun (the Photosphere) and will take approximately 5 1/2 hours to cross from solar limb to solar limb.
Time Lapse of Mercury Transit in 2016

If you want to see this, you will need special equipment that incorporates the correct level of filtering to reduce the Sun's intensity to a safe level, and magnification to make it possible to see the shape of Mercury against the disk of the Sun. Many astronomy clubs and public observatories will host viewing events. Here in Munich, the Volkssternwarte M√ľnchen will be open for the entire duration that the transit is visible from here, starting at 1:35 pm and continuing through sunset which is at 4:39 pm on Monday. We just need clear skies and a proper filter and we can enjoy this unique and rare event.

For general information about the transit and more detailed timing for US-based locations, check out this article from Space.com.

Warning: do not stare directly at the Sun for any length of time, and in particular do not look at the Sun through a telescope or binoculars with "eclipse glasses" as these will not protect your eyes sufficiently. Only observe the Sun directly with a high quality solar filter built especially for a telescope or binoculars.

Image courtesy of NASA.

14 October 2019

Cygnus and the Milky Way

Autumn brings us cooler evenings and earlier sunsets, but it’s not yet too uncomfortable to go outside and gaze up into the heavens for a look at our own galaxy, the Milky Way. The best way to enjoy this from a city setting is to get out to a place where you can lie flat on your back (on a blanket or lounge chair) and look directly overhead. In the evening the constellation Cygnus dominates the sky and is easy to find even with light pollution around you. Cygnus is also known as the Swan, as it resembles a giant swan flying gently toward the north-east and is also known as the Northern Cross, since its brightest stars outline a cross quite clearly. The brightest star of Cygnus is Deneb, a beautiful blue giant star that is one member of the Summer Triangle, an asterism that is made up of three of the brightest stars in the summer and fall skies. The diagram below shows the region of the sky directly overhead (zenith) with the three bright Summer Triangle stars identified in white, and the cross shape of Cygnus in purple.

Cygnus the Swan, or Northern Cross
Cygnus is directly in the band of stars of the Milky Way that we can see across the night sky. In dark conditions, the band of the Milky Way glows like a faint cloud but in the city that is unfortunately lost to light pollution. However, if you have binoculars or a telescope you can still enjoy the richness of the Milky Way by looking deeply into Cygnus where you will find many treasures that are quite accessible. I’ll cover some of those in a future post. For now, just see if you can get out and gaze up into Cygnus and into the elliptical arms of our own home galaxy. There are little gems and surprises awaiting you!

Image courtesy of Sky Safari.

02 October 2019

Time to restart - October 2019

It's been two years since I posted on this blog. It's time to restart. Since I started this blog I moved from San Francisco, California to Munich, Germany. Here the skies are slightly different and the weather is not nearly as reliable so I am observing less frequently. However, I still want to write occasional articles that describe the astronomical highlights in the night sky, in particular those accessible to city dwellers. More to follow ....