31 May 2020

15 minutes about: Our Place in the Universe

How can we possibly comprehend our place in the universe? I discuss this topic frequently during star parties and enjoy sharing my thoughts about how we can relate to the universe in a more tangible way. I created a 15 minute talk about this as part of an innovative science talk series in Munich, 15x4 Knowledge. The talk covers the various 'levels of scale' that define our universe, from our home planet Earth through the Solar System, Milky Way galaxy, the Local Galactic Group, to the Laniakea Supercluster. As the name of the series suggests, four speakers give 15 minute talks in a single the evening and it was an honor to be a part of their program. More videos from 15x4 Munich can be found on YouTube.

Click here to view the video "15 minutes about: Our Place in the Universe."

10 May 2020

Moon and Planets grace the morning sky

Moon and Planets - May 2020
This week, as the Moon passes through its waning phases, it will form some engaging patterns with three planets that are currently visible in the early (pre-dawn) skies. See image showing the change of location of the Moon from one night to the next, and the gentle interplay with Jupiter, Saturn and Mars as the Moon travels in its orbit around the Earth another 12 degrees eastward each day.

Jupiter has been gradually moving eastward as well, albeit considerably slower than the Moon. In fact, it takes Jupiter one full year (on Earth) to move the same 12 degrees eastward as the Moon does every day. And in addition, when we view Jupiter and the other outer planets from our view aboard spaceship Earth, we see a peculiar motion that takes Jupiter westward for a few months before continuing its eastward journey. We call this reverse motion 'retrograde' motion and in fact Jupiter is just now starting into retrograde and will return into the constellation Sagittarius for the summer before returning its eastward journey past Saturn into Capricornus at the end of the year.

This summer, Jupiter and Saturn will rise earlier and earlier and be visible the entire night from July onwards. But for now (in May) you will need to be up after midnight and before the break of dawn, around 4:30 am here in Munich, if you want to see this lovely morning alignment of the Moon and planets. I wish you clear skies and happy viewing!

Image courtesy Sky & Telescope.

26 April 2020

The Starlink Satellites

I have always considered satellites a part of observational astronomy. As an amateur astronomer, I enjoy simply looking up and trying to understand what I see in the heavens above. If a satellite happens by and I can watch its gentle arc for a while, I find that interesting and it enhances my enjoyment of being out under the stars.

Recently, SpaceX has been launching 100s of satellites into Earth orbit. The Starlink Satellite system will deliver high speed internet around the world. As a spectacle, I find the Starlink satellites fascinating to watch since they move in very large packs across the sky, not just single satellites but groups of them that move like a string of pearls across the sky for minutes at a time. I've observed these on many occasions over the past weeks. To see them, you will need a somewhat dark sky and a clear view especially toward the west. The website Heavens Above provides accurate information to help you locate times when the satellites will be visible based on your location on Earth.

Heavens Above - Starlink Passes
If you want to try to see the Starlink Satellites, your best bet is to find passes on Heavens Above when they will be appearing, which is typically within 1-2 hours after sunset based on your longitude and latitude. For city viewers, you will need to find passes that are at a brightness (magnitude) of 1 or 2. Higher numbers mean fainter satellites and they will be hard to spot. If you are in a darker location, you will be able to see passes down to magnitude 3 or even 4. Click on the image to the right to see an example, in this case based on my location in Munich, Germany.

The passes are quite impressive, with the graceful slow movement of a satellite from west to east being followed by a next satellite, and 15-20 seconds later another satellite, and so on. These chains of satellites are quite beautiful to see and if you are patient and look closely in the direction that Heavens Above advises, you will indeed find them. But you have to really pay attention to the cardinal direction (Azimuth, shown as east, west, north, south) and the height in the sky (Altitude, shown in degrees above the horizon).

There is controversy about these satellites. Because of the sheer quantity of them and the fact that they are bright enough to spot nearly every night, professional astronomers and astrophotographers are being disrupted in their work. SpaceX says they are working on ways to make the satellites less bright as they move up toward their final orbital altitude. The jury is out and we shall see how these satellites evolve over the coming months.

Postscript: on May 10th, 2020: SpaceX is working on plans to mitigate the impact of the Starlink satellites in the night sky, using a 'sunshade' approach.

02 April 2020

Venus passes through the Pleiades - April 2-3-4


Every eight years the path of Venus, as viewed from the Earth, crosses directly through the Pleiades star cluster. On April 3rd the bright 'evening star' will be directly in the center of the Seven Sisters, or Pleiades, and it will be a very fine target through binoculars. It can be found for the first 3-4 hours after sunset, due west. Venus is by far the brightest object in the evening sky so you should have no problem finding this.
Venus passes through the Pleiades

I just checked this evening (Thursday April 2nd) and already the juxtaposition of extremely bright Venus and the blue-white shimmer of the Pleiades is remarkable through my 10x50 binoculars. The good news is that the bright first quarter Moon to the upper left of Venus does not cause any issues seeing this spectacle, and given the brightness of both Venus and the Pleiades (as seen through binoculars) this will be visible even with the light pollution of a big city. But ... you will need binoculars to truly appreciate this.

The best view will be on Friday April 3rd, but it will also still look quite impressive on Thursday April 2nd and Saturday April 4th.

More information in this Sky & Telescope article.

Image courtesy of NASA APOD and Fred Espenak.

09 March 2020

The Moon: where is it going to rise tonight?

Moonrise in San Francisco
An old friend, Scott in San Francisco, posted a question on Facebook and it caught my attention. He illustrated a quandary that he faced when observing the moonrise in San Francisco on two consecutive nights and wondered why the two locations were so different. Here is what he wrote: "I am puzzled ... Can someone explain how the moon changes its position so radically in one day? I created a hand drawing of the moon, its location, and time as it rose over the San Francisco skyline the last two nights." There are two (well actually three) factors at work here.

Before we dive into the specific factors, let's review the basics. The moon, like the sun, rises in the east and sets in the west, but of course the precise point of sunrise varies throughout the year. The sun rises either north of east in the summer when the days are longer, or south of east in the winter when the days are shorter. The full moon, which Scott was observing when he sketched the images last month, has an opposite behavior compared to the sun ... that is, the moon is in the opposite part of the sky compared to the sun and as such, in the winter the moon rises north of east (when the full moon nights are longer) and south of east in the summer (when the full moon nights are shorter).

In addition to that macro change in the position of the moonrise compared to sunrise throughout a year, there are daily changes to the position of the moonrise. The moon, being much closer to Earth than the sun, moves rapidly in its 29 day orbit around the Earth and from one day to the next is approximately 12 degrees further eastward in comparison to its location the night before. That is the main reason that a rising moon rises approximately one hour later each evening, and the primary factor that is illustrated in the drawing above. The Earth has to turn more to get to the place where the moon has moved, in this case from 8:00 pm to 9:00 pm/.

Moonrise position changing
But there is a subtle secondary effect that Scott captured in his sketch. That is that the location of moonrise is quite a bit shifted on the horizon. In this case, it is to the south from one day to the next, not just a later moonrise time but also a point on the horizon that is south of the previous day. What is happening here is that the moon's path around the Earth is quite similar to the sun's path around the sky and that is a path that follows the zodiac band around the sky, something also called the ecliptic. Because the moon follows this path, and in this special circumstance in which a full moon is observed on successive nights, the winter full moon appears to rise further and further south on consecutive nights until it finally moves to last quarter moon phase and then it will resume its travels northward (north of east) as it rises late each night. In fact, the moon is simply moving across the known Zodiac constellations of Virgo, then Libra (see image on left), and eventually Scorpius and Sagittarius, all of which are low on the horizon and low on the ecliptic, and therefore are part of the reason why the moon rises further and further south of east on consecutive nights in the winter. The image shows how the moon moves across the Zodiac from night to night, but it will take some creativity to juxtapose this on the eastern horizon to fully understand why the moon is further south of east on consecutive nights. Think about it. It requires some big picture thinking.

There is a third factor for the change in the location of moonrise on consecutive nights which is based on the moon's orbital inclination, but that requires fairly expert knowledge of celestial mechanics to fully comprehend so we will save that for another time ... or a star party.

Today is the full moon, March 9th, 2020. You can try to reproduce Scott's experiment now, looking closely at the eastern horizon and locating landmarks where you can spot the moonrise on consecutive evenings. For the next 3 or 4 evenings the moon will rise about an hour later each evening, and also will rise at a point further to the south of east compared to the night before.

I wish you clear skies and happy viewing.

02 February 2020

TED Talk: How looking up to the night sky gets us to think deeper

As an amateur astronomer I have hosted many star parties, and as a public speaker I have talked about the topic of astronomy in many settings. In November 2019, I had the privilege of giving a short talk about astronomy at the TEDxTUM event in Munich, held at the Technical University of Munich. The talk is about 9 minutes long and captures the essence of why I do a lot of public-facing astronomy events. I hope you like it.

22 January 2020

The Spectacular Winter Circle

Winter brings out the best in the night sky. Some of the finest stars in our galaxy are concentrated in a region of the sky that shines with an intensity that you just can't miss on a clear night. A collection of extremely bright stars form a nice shape that is fun to look for, the Winter Circle. This asterism features 5 of the 8 brightest stars in the entire night sky, and in total 7 very bright stars that glow in the south on long winter nights.
The Winter Circle

Given that all of the stars in the Winter Circle are bright, this is a relatively easy target for city dwellers who might not normally pay attention to the night sky. The stars are part of six different constellations covering a fairly significant part of the winter sky. January evenings feature the Winter Circle moving gradually from east to west across the southern horizon. Given the span of this asterism from the lowest point at the star Sirius (see diagram) to the highest point at the star Capella, you will need a fairly clear view across the southern sky. For me, it's easy to spot one or two constellations wherever I am, but seeing all six constellations and all 7 of the bright stars in the circle requires that I have a good horizon and not much else blocking the way.

Most people can quickly and easily find the constellation Orion, and from there you can start your trip around the circle. Rigel is a blue supergiant star that is one of the kneecaps of the hunter Orion, and it is one of my favorite stars because of the intensity and color. Orion's 'belt' can help you find two other members of the circle, by following the belt up toward Aldeberan and down toward Sirius. The other stars are then easy to find looking up and left of Orion toward Gemini, Auriga and Procyon.

Finding a wide open space on a clear winter night where the grandeur of the Winter Circle is on full display is one of my favorite things. The stars are rich and intense, and the utter size of the circle make the viewing of this amazing pattern worth the effort. Dress warm and savor the moment. You will be glad you made the effort.

Image courtesy of SkySafari.

Update: good article from Sky & Telescope on the Winter Circle. They call it the Winter Hexagon.

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.