Not so Blue Moon!

I recently encountered a number of questions about this month's double full moon. The first full moon occurred on 2nd August (2012) and the second is tonight (Friday 31st August).
Its a common misconception that the second full moon in a month is called a "blue moon". I also made this assumption previously, however this is not the case. Having two full moons in one month does not result in a Blue Moon.
Throughout several conversations this week, I have attempted to explain why and this is the best explanation I can come up with:

The rather long background information
Our 12 month Gregorian Calendar is partially based on the 12 Lunar Cycle that often (but not always) occur in one year but individual calendars months (like August) are no longer in sync with the (approximately) 29-day lunar month (i.e. the time between two New Moons).
This results in some months (e.g those with 31 days) having two Full Moons or two New Moons.

The term "once in a Blue Moon" is not related to the number of Full Moons in one month but instead, the number of Full Moons or lunar cycles between two seasons.

Most calendars around the world have significant dates that represent the start of seasons. These dates are based on the Sun's position in the sky.

For example, in European calendars, the "Start of Spring" was usually celebrated on the 20th Mar (or 21st Mar), when the Sun is exactly level with the Earth's equator. This is referred to as Vernal Equinox where day and night are equal in length (Equi -"equal"; Nox -"night").
Likewise, the "start of summer" is usually 21st Jun or Solstice, when the Sun reaches its highest position in the sky, resulting in the longest daylight hours.

In Asian calendars, the "start of spring" (Lichun, 立春) is actually marked by the halfway point between the December Solstice and Vernal Equinox, i.e. when the Sun is 45 degrees behind (or 315 degrees in front of) its Vernal Equinox position.

The actual answer
How does the Blue Moon fit in to all of this?
Most of the time, there are three Full Moons between Equinox and Solstice (i.e. between seasons). But occasionally when a Full Moon occurs on or near the Equinox or Solstice, there will four Full Moons between the seasons. The third moon of a four moon season is called the "Blue Moon" (It will take another essay to explain why).

Consequences and Complications
Having an extra Full (Blue) Moon between seasons makes things a little bit messy, because cultural and religious festivals like Easter and Chinese New Year are based on lunar and seasonal dates.
For example, Chinese New Year is always celebrated on the New Moon closest to Lichun (the start of spring based on the Sun's position).
When there is a Blue Moon between seasons (like in 2013) it can result in the date of next Chinese New Year moving back (e.g from February to January) by almost half a month. Over several years, this gradual shift can result in Chinese New Year moving into winter months like December or November.

To accommodate this, extra/double months have to be added into the Chinese Lunar Calendar during certain years in order to push forward significant festivals, e.g. so that Chinese New Year stays close to spring/LiChun.

Phew ... that explanation was a little bit more complicated and longer than I thought!

7 useful astronomy resources

Finding out what's happening up in the sky often involves using a variety of resources, such as books, star charts, astronomy software, apps, etc. Some may present you with a bunch of number or unfamiliar terminology. Here's some of my favourite online resources, which are relatively easy to use and interpret as well as some new ones which I just recently discovered.

1. Sky and Telescope: Interactive Sky Chart (JAVA applet)

This popular astronomy magazine has many useful features on its website. The Sky Chart provides a relatively simple view of which stars and planets you can expect to see at any time of day or year.

After registering your details on the site and opening the sky chart, you first have to set your location/hometown and timezone (i.e. Singapore, GMT+8). Once that's done, you can alter the time and date on the side panel and watch which objects appear during that particular time.

2. Skymaps.com

This website provides a static but more detailed starmap for each month, in an easily printable pdf format. The important part is to pick the right map for your part of the world, i.e in Singapore, you'll need the Equatorial Edition.

3. Moon Phase Images

Enter your desired date and time, and receive a simple image of the moon phase on that particular date.

4. NASA Solar System Simulator

View any object in the solar system as they would appear from any other object. For example, look at the rings of Saturn as seen from its largest moon Titan or how the Sun and other planets look like from Pluto.

I often use it to get an overall view of the whole solar system, to see the arrangement of planets during significant alignments or the position of current space probes, like New Horizon, which is on its way to Pluto.

On the menu page, you need to select the object you wish to view and  where you want to view it from, as well as the size of the field of view.

NASA Solar System  Simulator -  field of view 45 degrees - wide enough to see Pluto, but too far to see inner planets

NASA Solar System - field of view 5 degrees - only wide enough to see up to Jupiter. but can show position of inner planets.

5. Dynamic Diagrams Solar System (SWF/FLASH)

I recently remembered this rather nice-looking solar system model. Although the size and distances of the planets is not to scale, their positions are accurate.

Just drag the arrow indicating the date around the outer edge or select the running speed on the control bar in the top left corner and watch the planets dance around each other.

The two options at the bottom right corner (Copernican and Tychonian) give two slightly different views: Copernican shows the more realistic view with the Sun at the centre, whereas Tychonian puts Earth at the centre, allowing you to see the position of planets as seen from Earth.

6. The Planets Today

Very similar to dynamic diagrams, except with more options that allow you to adjust the size and scale of the planets. 

Both Planets Today and Dynamic Diagrams appear to be used for astrology, as they provide information regarding the position of the planet's with respect to zodiac star signs (Leo, Gemini, etc). Whilst the zodiac does represents actual constellations, the position of the astrological star signs given in these two sites is scientifically inaccurate and out of date. However, you have to option to remove the zodiac star signs from each diagram.

Finally, for a bit of solar system exploring fun try Ace of Space and pilot your space ship around the solar in the fastest possible time.

Mars and Saturn align

Last night (14th August) was another great opportunity for planet watching, as Mars and Saturn lined up in the early evening with Spica, the brightest star in Virgo.

Just as I arrived home, after a late event at Science Centre, I saw them peeking through a gap in the clouds. I only had a couple of minutes to snap a few shots before thick clouds covered them again.

From left to right: Spica (star of Virgo), Mars, Saturn - as seen from Singapore on 14th August 2012 9pm

Over the course of the next month, all three objects will gradually get lower before finally disappearing below the horizon as Earth moves further and further away towards the other side of the Sun.

As this happens, Mars will also move westward (upwards) away from Saturn and Spica.

Diagram illustrating the position of Mars, Saturn and Spica as seen from Singapore and other equatorial regions during August 2012

This alignment was due to how Earth, Mars and Saturn move around the Sun.

As seen from Earth, Mars and Saturn are currently directly in front of the more distant stars of Virgo. Earth and Mars orbit the Sun much faster than Saturn, which results in Mars appearing to move closer to Saturn before overtaking and finally moving away from the ringed planet. 

Watching for Perseids...... and early morning planets

Was awake at 4am last night to watch the Perseid meteor shower. Backed out of my grand plan of staying up all night in favour of getting some sleep for the first half of the night. Unfortunately, as is often the case, the sky was too cloudy and bright. In the end I only saw two bright meteors, one about 4:45am, the other around 5:30am.

Our other meteor shower events in previous years didn't yield much results either. Of course, Singapore, like most cities, isn't a great place for watching meteor showers but it was worth a try. Every meteor shower I've watched here has turned out cloudy for most of the night. In fact the last meteor showers I watched whilst studying in the UK, also turned out cloudy, although I did see more meteors overall.

Anyway, as the clouds began to clear, towards dawn, I hung around to gaze upon the rather nice sight of the Crescent Moon, in-between Juipter (above) and Venus (below) together with several constellations like Taurus and Orion: 

The "M-shaped" constellation, Cassiopeia , faintly visible through clouds.

Highlights for August 2012

Its already the middle of August and so much is happening up in the heavens.

Planets in August
Last monday, NASA successfully landed  the largest robotic rover, Curiosity, on Mars.

In the sky, Mars is still visible low in the west, during the early evening, forming a neat little triangle with the bright star Spica and the planet Saturn. This triangle continues to get smaller every day, until about 14th August when the three objects appear in line with each other. After 14th August, Mars will gradually move upwards away from Saturn and Spica. They are best viewed before 9pm.

The reason for this alignment is that Earth, Saturn and Mars are roughly in line with each other in the Solar System. However, the three planets are not in line with the Sun, as Mars and Saturn are located towards one side of Earth, which is gradually speeding away towards the other side of the Sun.

There is also a similar alignment towards the morning side of the Earth, with Venus and Jupiter still visible in the early morning around 6am.

Meteor Showers
August is also a great time for meteor showers as the not so prominent Delta Aquarids comes to an end at the beginning of August and the much more obvious Perseids Meteor Showers peaks around 12th and 13th August. To view the Perseids its best to stay up all night and watch the sky between midnight and dawn on either 11th or 12th August to have the best chance of catching shooting stars. However, in an urban environment like Singapore, you're likely to see much less meteors than those in darker areas.

Early morning (6am) on 13th August, you'll also see the waning crescent Moon just above Venus and Jupiter. With four planets, the moon and a meteor shower visible in one night, I'm hoping to stay up all night on Sunday and take some photos.

Stars and Constellations
As in July, Scorpius and Sagittarius remain high and bright in the sky, towards the South. Here's a recent photo of Scorpius taken by one of our scobbers, Li Fei:

On 24th August there is a First Quarter Half Moon, this night is also known as Qi Xi 七夕, the 7th day of the 7th month of the Chinese Lunar Calendar. Associated with this night is the story of Niu Lang 牛郎 (cow herd) and Zhi nu 织女 (weaver girl), who become separated and meet again every year on Qi Xi.
The story is also represented by the constellations Cygnus, Lyra and Aquila. The brightest stars of these three constellations (Deneb, Vega, Altair) form the Summer Triangle which appears low in the east this month.

Recent Planetarium Shows at Science Festivals

Trying to catch up on some observatory updates after all the education programmes and events that have been going on since the start of the new school term at the end of June.

We've just finished a week of Kids Science Fest, as part of the Singapore Science Festival, where amongst activities such as light painting and marble machines, we also conducted six planetarium shows a day inside our STARLAB portable planetarium.

 These planetarium sessions are normally available for bookings by school and kindergarten groups but this is the second time we've used the STARLAB at this event, in addition to a two week period during the December holidays last year.


The system uses a popular astronomical software called Starry Night, which displays a virtual image of the sky via a projector on the inside of the inflatable dome. The projector is equipped with a fisheye lens to create a 360 degree view of the entire sky.

We actually have another planetarium that we take out of Science Centre to schools as part of our outreach programme. Last month, we visited Anderson Secondary School and Victoria Junior College.

This older more basic system, uses a halogen light bulb and series of projecting cylinders, each of which has a fixed number for images, for examples e.g constellations.

Here we are set up on a sheltered basketball court at Anderson Secondary, as part of their local cluster science day event, which involves a number of other schools from the around the same area.

Ideally, these portable domes should be placed in a indoor, air conditioned venue. However, due to limited space at the school, we opted for this semi-outdoor area, which although sheltered from Sun, was still very warm and stuffy inside the dome as warm outside air is continually blown in to the dome to keep it inflated.

A few days later we were at VJC, for a similar Science day event for east zone schools. We've attended this event for the past few years and set up in our regular venue, the AVA Lecture Theatre. There's just enough room on the small stage to squeeze in the fully inflated dome ad there's air-con too!!!

It has crossed my mind to set up an inflatable planetarium at the observatory, on several occasions during Friday night stargazing, however, we're a bit short on space as well as manpower. A more permanent planetarium may be the answer, but that may take sometime to become a reality, we'll see what we can do.

Photos of Stars and Planets - July 2012

Things at SCOB have been a bit quiet this month in comparison to a very active June holiday period. Weatherwise, the first two weeks of July were full of cloud and rain and we didn't get to see much during Friday night stargazing.

However, hot weather did return and with it, clear skies. The last two stargazing sessions were very fruitful as we got to see several of those bright star clusters that I was talking so much about in an earlier post.

Open Star Cluster NGC 6231 located in the constellation Scorpius - as seen through a 6-inch refractor telescope.

Unfortunately we didn't see any of those fainter globular cluster but clear skies meant that we could also see a rather nice display of planets. In the early evening Saturn, Mars and a star called Spica formed a bright triangle in the western part of the sky. This triangle will gradually get smaller over the next weeks as Mars gets closer to Saturn.

Last week I also managed to wake up early in the morning at 6am, to go outside and take some photos of another bright trio of objects. After a few failed attempts, due to clouds, my sleepy eyes were suddenly revived by the sight of Venus, shining brilliantly white, along with Jupiter and a star called Aldebaran (in the constellation Taurus) along with a bunch of other familiar stars, including the constellation Orion. 

Venus (brightest and lowest), Jupiter (top left) Aldebaran (star, right of Jupiter) as seen from Singapore - 6:30am, before sunrise in late July.

Whilst at home, I also took advantage of the clear skies and ventured outside on a few nights after dinner to catch another glimpse of some bright constellations. Once the moon had gone, many bright stars stood out, including the constellations Scorpius & Sagittarius.

The teapot shape of constellation Sagittarius (left) and hook-shaped constellation Scorpius (right) - as seen in a light polluted area of Singapore.

I find it very enjoyable to look at constellations with just my eyes or a pair of binoculars, especially with clear skies.  After a while though, it was time to leave the stars and the mosquitos behind and head back inside, where I joined my wife to watch some indoor stars, i.e. watching Korean drama.......An-nyeong-ha-se-yo!