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.

10 December 2019

Geminid Meteor Shower 2019 & Full Moon

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