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Posts Tagged ‘the Moon’

Last week, two more Chinese astronauts (or “taikonauts” as they are sometimes known) blasted into space, to spend a month on-board China’s experimental space station Tiangong. They successfully docked with the space station just before 19:30 GMT last Tuesday (18 October). The 30-day stay on the space station will be the longest mission yet undertaken by Chinese astronauts.

This is the latest chapter in an ambitious space programme; China has plans to send manned missions to both the Moon and Mars, although it has not publicly stated a time-line for these two goals. In fact, nothing would boost China’s feeling of becoming the World’s premier superpower than if they were to get to Mars before the USA.

The pace of China’s space programme is impressive. They are spending some US$2.2 billion a year on it, and to-date have sent 11 people into space. They plan to build a permanent space station by 2020, and have already launched 181 satellites into space.

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A summary of some of the key numbers for China’s ambitious space programme.

In 2016 alone it will have launched 20 space missions. I have heard it argued that it is easier for a one-party state like China to achieve ambitious long-term programmes like exploring space than it is for democracies like the US. This is because any programmes suggested and funded in the US can be axed by Congress, or shelved by a new president. Such changes of government do not happen in China. Of course, it is looking increasingly likely that the first US manned mission to Mars will not be undertaken by NASA, but rather by one of the private companies like Space X.

The race is on to get to Mars first, who do you think will be first?

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On Tuesday morning of last week (the 3rd) I woke up early, which is a habit I am trying to get back into so that I can recommence my running, which I try to do first thing in the morning before work. In a semi-awake state I was listening to BBC Radio 5’s breakfast show co-presenter Rachel Burden and someone else on the show between 6 and 7 (was it George Riley the sports person or someone else??) discussing how they had seen the Moon and Jupiter very bright in the sky on Monday evening.

The following morning during the same 6-7am period, the discussion was resumed with remarks being made about how much Jupiter had moved away from the Moon. I decided to tweet into the show and to Rachel saying that it was the Moon which has moved, not Jupiter. To cut a long story short, I ended up being on the show on the Thursday morning to explain what was moving and why. Here is a link to a recording of my 3-minute stint, where Rachel and Nicky Campbell (Rachel’s co-presenter) interview me.

The evening before my being on the show, one of the show’s researchers had asked me to send in a list of the five most interesting facts about Jupiter. They then put those out as a tweet during my interview. I chose the following five, would you have chosen the same ones?



The tweet from BBC Radio 5 with the five most interesting facts about Jupiter.

The tweet from BBC Radio 5 with the five most interesting facts about Jupiter.




The list of the five facts

The list of the five facts



I will talk about each of these five facts separately in future blogs over the next several weeks. I will also blog in a little more detail about the NASA mission to Europa and the proposed ESA mission Juice; both of which intend to study this fascinating moon in more detail.

Being on the radio it is, of course, impossible to show diagrams about the motion of the Moon and Jupiter; and there really wasn’t enough time to explain it properly. So, I have decided to put together these slides to explain it in a little more detail.

Jupiter and the Moon during the first week of February 2015

Getting software to show you what is in the sky is easy, and although for PCs and Macs you may end up paying several tens of pounds, for tablet devices the software is much cheaper, with many reasonable ones being free. I use “Skysafari”, which is not free but is not too expensive either. It is made by Carina Software, who made the wonderful Voyager Software on Macs that I used for many many years in my classes.

Below is a screen capture using this software of the sky as seen from London at 20:00 on Thursday the 5th of February 2015. As you can see, I have done the screen capture with Jupiter just to the left (East) of the middle of the window (in the software you can use your finger to move around and look in different directions such as north or north-east if you wish). The Moon is at about 7 o’clock from Jupiter in direction, if you imagine a clock face.



This is a screen capture from an app I use on my iPad called "Skysafari" which can show what is in the sky at any location and at any time and date. There are lots of other similar apps available, but I like this one the most of the ones I've tried. This is the screen capture looking south for 20:00 on Tuesday the 2nd of February 2015 as seen from London

This is a screen capture from an app I use on my iPad called “Skysafari” which can show what is in the sky at any location and at any time and date. There are lots of other similar apps available, but I like this one the most of the ones I’ve tried. This is the screen capture looking south for 20:00 on Thursday the 5th of February 2015 as seen from London.



Below I show a sequence of screen captures of Jupiter and the Moon (I have zoomed in on just enough to show the two) from Monday evening (the 2nd) to Friday evening (the 6th), all at 20:00 to show the motion of the Moon compared to Jupiter’s position.

Jupiter and the Moon as seen on xx of February at 20:00 from London

Jupiter and the Moon as seen on Monday the 2nd of February at 20:00 from London

Jupiter and the Moon as seen on xx of February at 20:00 from London

Jupiter and the Moon as seen on Tuesday the 3rd of February at 20:00 from London

Jupiter and the Moon as seen on xx of February at 20:00 from London

Jupiter and the Moon as seen on Wednesday the 4th of February at 20:00 from London

Jupiter and the Moon as seen on xx of February at 20:00 from London

Jupiter and the Moon as seen on Thursday the 5th of February at 20:00 from London

Jupiter and the Moon as seen on xx of February at 20:00 from London

Jupiter and the Moon as seen on Friday the 6th of February at 20:00 from London



I think these screen captures make it quite easy to see that Jupiter is staying fixed in the same place relative to the stars during this sequence (which spans 5 nights), and it is the Moon which is moving. Why is this?

The Motion of the Moon in the sky

The reason it is the Moon which appears to move against the background of Jupiter and the stars is because the Moon is orbiting us; whereas Jupiter is orbiting the Sun and the stars are not orbiting the Sun but are, along with the Sun, in fact orbiting the centre of our Milky Way galaxy.

As the Moon takes roughly 30 days to orbit the Earth (see this blog for the more precise figure, and the difference between how long it takes to orbit the Earth – the “sidereal month” – and how long it is between two New Moons – the “synodic months”), then if we divide 360^{\circ} by 30 we get that the Moon moves 12^{\circ} in its orbit about the Earth each day/night. This figure is only approximate (but good enough for our purposes) because (a) a sidereal month is not exactly 30 days and (b) the Moon moves in an ellipse and not a circle about the Earth, and so changes its speed at different points in the orbit, so does not move the same amount each 24 hour period.

As the Moon is 0.5^{\circ} in diameter, 12^{\circ} corresponds to 24 times the diameter of the Moon. This is quite a lot, and so the motion of the Moon from night to night against the background planets and stars is very easily seen, as the sequence of diagrams above show.

In fact, Jupiter is also moving against the background stars, but it does so much more slowly. Jupiter takes about 12 years to orbit the Sun, and so each year it moves roughly 1/12th of a full circle. Along with the Sun, the Moon and the other planets, Jupiter moves through the zodiacal constellations during its travels, and so moves roughly into a new zodiacal constellation each year. At the moment it is in Cancer, but by this time next year it will be in Leo, the next constellation along the zodiac to the East (to the left of Cancer in the diagram above). You may be able to notice that Jupiter has moved relative to the background stars in a month or two, but certainly by next February, if you remember where it is now, you will see a difference.

Finally, although we refer to the stars as “the fixed stars”, they are not fixed. They, along with our Sun, are orbiting the centre of our Milky Way galaxy. Our Sun will take 250 million years to do this, stars closer to the centre of the Milky Way will take less time and stars further out from the centre will take longer. This leads to the positions of the stars relative to each other changing, but the change is very very slow, taking tens of thousands of years to be noticeable.

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Last week, the 9th of November 2011, marked the end of Eid al-Adha, the Muslim festival to commemorate Abraham’s willingness to sacrifice his only son Isaac to God. Eid al-Adha started on Sunday the 6th of November (or Monday the 7th, depending on where you are) and ended on the 9th (or 10th) of November in 2011. But, next year it will start on Friday the 26th of October, and last year (2010) it started on the 16th of November. Why does it move?

It all has to do with the Moon. For the Muslim calendar, which is based on the Moon, Eid al-Adha is celebrated on the 10th day of the 12th month, so on the 10th day of the month of Dhu al-Hijja. There are 12 months in the Muslim calendar, and each month lasts from one new Moon to the next new Moon.

A Waning gibbous Moon

A waning gibbous Moon

I got this quote from here

Regional customs or moon sightings may cause a variation of the date for Islamic holidays, which begin at sundown the day before the date specified for the holiday. The Islamic calendar is lunar and the days begin at sunset, so there may be one-day error depending on when the New Moon is first seen.

Most societies initially created calendars based on the Moon. If you think about it, there are only three natural cycles, apart from the daily one of day and night. These are

  • the waxing and waning of the Moon
  • the time it takes for the stars to appear in the same part of the sky at e.g. sunrise.
  • the solar cycle, e.g. the time between successive longest days of the year.

Of these 3, the cycle of the Moon is by far the most obvious and easiest to observe, and it is why most early civilisations based their calendars on the Moon. Today, most calendars are either Solar (based on what the Sun is doing), or Lunisolar, a combination of Lunar and Solar calendars. Examples of lunisolar calendars are the Jewish calendar, the Chinese calendar, the Hindu calendar and even parts of the Christian calendar, such as the date of Easter [which is determined by a combination of what the Sun and the Moon are doing]. Any festivals which are based on a Lunisolar calendar will move from year to year, but back and forth rather than just getting earlier each year.

The Moon actually takes 27.32 days to complete a 360 ^{\circ} passage around the Earth. This is known as the sidereal period of the Moon, the word sidereal deriving from the Latin word “sidus” meaning “star“. So the sidereal month is the completion of an orbit with relation to the “fixed” stars [so, if you were in a space ship looking down on the Moon moving around the Earth, with the distant stars in the background to provide a reference, you would see the Moon move 360^{\circ} in a sidereal month].

However, this is not the time between one new Moon and the next, or one full Moon and the next. For a second new Moon to occur, the Moon has to travel a little further than 360^{\circ} in its orbit. This is because, in the time between the previous new Moon and this one, the Earth has moved around the Sun, and so the Moon has to travel a little further than 360^{\circ} to produce a 2nd new Moon.

A sidereal and synodic month

The difference between a sidereal Month and a synodic Month

This takes 29.53 days, and is called the Synodic period of the Moon. [Note: due to variations in the Earth-Moon system, and the fact that the Earth varies its speed of orbit about the Sun during the course of the year, the synodic period varies between 29.18 and 29.93 days. 29.53 is the average.]

There are nearly exactly 365.25 days in a year [I will come back to discuss the measurement of what a “year” is in a future blog], so if you divide \frac{365.25}{29.53} you get 12.37, which is not exactly 12, so a 12-month calendar based on the Moon will not fit into a year without some days being left over. The civil calendar in all(?) countries is the Gregorian calendar, which keeps the Sun doing the same thing on the same day each year (i.e. it is a solar calendar). The number of extra days between a 12-month Lunar calendar and a year is 0.37 \times 29.53 = 10.89, so nearly 11 days. In a 12-month Lunar calendar, the same day in the same month will be approximately 10-11 days earlier each year.

This is why Eid al-Adha started 10 days later in 2010 than it did this year, 2011, and why Easter will not be on the same day in 2012 and it was in 2011, and why the Chinese new year will not be on the same date in 2012 as it was in 2011, or Diwali, or Hanukkah.

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