BLUE MOON

It won't actually be blue. But that is what some call the second full moon to occur within a month. This is only a recent interpretation.  

Since the moon is full once every 29 and a half days, there is usually one each month. Two in one month is a little unusual, but it's happening in September, or in August depending on how you look at it.

The moon will be at its fullest at about 2am on the morning of September 1.

You would be right in thinking that the moon will look more full on the night of August 31 than on the night of September 1.  

You would also be correct in noting that for most of the planet, that moment of full moon will occur on August 31 their time.  

That would make it the second August full moon for them, and therefore a Blue Moon. However, for us Blue Moon won't occur until September 30 our second full moon in the month of September. 

However, a more traditional true blue moon - also not actually blue - occurs under slightly more complicated circumstances.  

Divide the year into its seasons, each of three months. Seems simple enough.

When a season will have four full moons occurring in it, then the third one is properly called a blue moon.But this historical definition of Blue Moon is fraught with its own international difficulties.  

The astronomical seasons, accepted in many if not most countries, begin and end on the solstices and equinoxes.

The spring equinox is September 23 in New Zealand, the beginning of astronomical spring.  

But in NZ, spring is held to begin on September 1. That would make October's full moon the third one this spring, and November's the fourth.  

By that rule October's one full moon would be called Blue for us.

But in the United States for example, the spring season will hold only three full moons, so no Blue Moon at all.  

My advice is to just enjoy the full moon whenever you have the chance to see it.  

It is even more beautiful with a few clouds around to reflect its silvery light.

MARS ON THE MOVE

We have been watching Mars, along with the background of stars, moving nightly from east to west across our sky for a couple of months now. 

This motion is the result of Earth's daily progress in our orbit around the Sun.

And yet, compared with the stars it is travelling with, Mars has been moving in the opposite direction. 

This is especially clear when we look at Mars' position with respect to the star Spica and also Saturn, Spica's travelling companion of the same few months. 

We marked their positions in the sky just a month ago, at the end of July.  Mars was west of, and well below, Saturn and Spica, setting a couple of hours before them. 

Look toward the west this week after dark and you will see that July triangle has been completely up-ended, with Mars now west of and well above, the other two.

The stars each have their own path and speed around the centre of the galaxy, however, they don't noticeably change their positions in the sky during our lifetimes.

The planets, on the other hand, appear to wander continuously amongst the stars. 

This caused ancient Greek astronomers to refer to them as planetes asteres, "wandering stars". 

This wandering is because each planet is moving, in its own time, around the Sun. It's as if you added more hands to a giant clock, hands of different lengths and moving at different speeds (except the shorter the hand is, the faster it circles the face of the clock).

The recent apparent speed and movement of Mars, swiftly moving past Saturn and Spica in the sky, are simply the result of the relative clock-like motion of Earth and the other planets with respect to each other and to the stars.

Meanwhile, as we enjoy the celestial show, Mars and Saturn each simply continue their smooth movement as they orbit the Sun.

- © Fairfax NZ News

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