When is the moon full?

If you have never watched the full moon rising above the hills on the other side of a lake, you really need to do something about that.

It is beautiful and special. An added mystique comes from knowing we are doing something even our remotest ancestors must have done.

This may be one of the reasons the phases of the moon are listed in most of our diaries and calendars. However, if so why are the dates of the lunar phases sometimes wrong, usually out by a day?

Our ancestors must have taken note of the phases of the moon, although they did not know what was going on. They reckoned the passage of time using the moon. That is where the word "month" came from.

Today, we do understand what causes the lunar phases. They arise from the direction we are looking at the moon compared with the direction of the sunlight illuminating it. We can only see the moon because of the sun lighting it up.

In fact, unless sunlight is being reflected from the Earth onto the unlit part of the moon, a phenomenon known as the "Old Moon in the New Moon's arms,” we only see the sunlit portion.

Imagine you are sitting way out in space, way above the Earth's North Pole. You will see the moon circling the Earth like a ball on a string. The half facing the sun is lit up and the other half dark and invisible.

When the moon lies between the sun and us, the unlit side is facing us. In addition, when the moon is in that position, it is close to the sun in the sky and it is totally lost in the glare.

We refer to the moon at this time as being new.

The path of the moon around the Earth is in the eastward direction. So it moves leftward from the sun until a short time after being new, when it appears as a thin crescent in the western sky after sunset.

What we are seeing is a small sliver of the sunlit half. Many amateur astronomers take on the challenge of seeing the youngest moon possible, as a thin thread of crescent in the sky just after the sun has gone.

From that point, every day the moon will be about 12 degrees further east and rise about 50 minutes later. During this part of the cycle, the waxing moon is lit from the right, so it looks like a "D".

Around a week after the New Moon, the disc will appear exactly half lit, exactly like a "D". The moon is now at First Quarter.

After about another week, the moon will be at the opposite side of its orbit from when it was New. We will be between it and the sun and it will be lit from over our shoulders. We are then facing the fully lit side: the Full Moon.

It will continue on its orbit, over following days we will se an increasing fraction of the unlit part of the moon appearing on the right hand side. The moon is now waning and looks rather like a letter "C".

A week or so later, we will see the disc half lit, except this time lit from the right hand side. We are now at Last Quarter. From then on narrower, until the moon is again New. A simple guide is DOC: D-waxing, O-Full and C-waning.

The times and dates of the phases of the moon can be calculated accurately. We usually get this information in Universal Time (which used to be called Greenwich Mean Time). For example, on Jan. 21 2019, the moon was full at 05:16 UT.

Unfortunately, this is 12:16 a.m. EST on the 21st in Ottawa, but 09:16 p.m. on the 20th in Vancouver.

In general, most of the calendars we buy are printed in Europe or in eastern North America, with the dates of the phases of the moon given for those locations. Those of us living elsewhere have to live with the discrepancy.

This is not really a problem. We can get the exact times from the Web, or from the Observer's Handbook of the Royal Astronomical Society of Canada.

  • Jupiter and Saturn lie low in the southwest after dark.
  • The moon will be new on the 28th (in Eastern Canada) and the 27th (in the West).
  • Mars lies very low in the east before dawn.

Saturn is the moon king

Researchers using the Subaru Telescope in Hawaii have found 20 more moons orbiting Saturn.

This brings the total orbiting the planet to 82, overtaking Jupiter, for which the total discovered so far is 79.

These new satellites are all about five kilometres in diameter, and discovering them is an indicator of the quality of modern telescopes.

Our Earth has one, quite large moon. Mercury and Venus have none, and Mars has two.

However the giant planets, Jupiter, Saturn, Uranus and Neptune have large collections of moons, and as our instruments improve and our spacecraft pass by, we are finding more.

Is this due to the planets actually acquiring new moons, or is this simply due to improvements in our instruments?

Thanks to our having a number of telescopes watching for them, almost every year we notice an asteroid passing us close by, sometimes closer to us than the moon.

Even when this happens, and the Earth's gravitational attraction on that asteroid is stronger than the Earth's pull on the moon, the asteroid does not get captured; it passes by and continues on its orbit around the sun.

The reason is simple. As the asteroid approaches the Earth, it accelerates due to the Earth's pull. When it swings past the Earth, it will be going fast enough to escape into space with the speed it had before it encountered us, although it will probably be moving in a different direction.

To be captured, it needs something to slow it down a bit. The empty vacuum of space has nothing in it to do that, which is why our Earth is circled only by the moon, satellites we have launched and a growing accumulation of space junk.

There might be a tiny extra moon out there that we haven't found yet. The same applies to the giant planets, Jupiter, Saturn, Uranus and Neptune.

Anything orbiting the sun that flies past them will head back off into space. However, it has not always been like that.

The solar system formed from the collapse of a big cloud of gas and dust. It collapsed into a rotating disc, with the centre becoming the sun. In the remaining disc material, smaller discs formed,  which collapsed to form the planets and their moons. These moons orbit in the same direction as the planets' rotation.

For example, our Earth rotates eastwards, and that is the direction of the moon's orbital motion. While any remaining disc material is present, an object orbiting the sun and coming too close may be slowed down enough by that material to be captured, becoming a new moon.

This is most likely for the giant planets, because they formed from large discs, which provide particularly large "catcher's mitts.” 

These outside additions can end up in any sort of orbit and can even move in the opposite direction to the other moons. Seventeen of Saturn's moons have these reverse or "retrograde" orbits, suggesting they are external additions.

In the outer solar system, around the giant planets there are opportunities for passing objects to seem to be captured but actually aren't. If the encounter is at the right speed and the right distance from the planet, an object might make an almost complete orbit before wandering back off into space.

This apparent orbit might be many years long, so it will take time and some careful measurements to find out whether an object is a distant moon or a visitor.

The data for these new Saturnian moons was collected between 2004 and 2007, which produced enough information for some new computing algorithms to derive orbits for the new discoveries.

During coming years, as our telescopes and calculations improve, we expect we will be finding even smaller moons, maybe as small as one kilometre, and the count will go up.

  • Mercury and Venus might be visible very low in the southwest after sunset if the sky is clear.
  • Jupiter and Saturn lie low in the southwest after dark.
  • The moon will reach Last Quarter on the 21.

Black holes shred stars

The term "tidal disruption event" or TDE must be one of the biggest cosmic euphemisms around.

This is what we say is happening when a star is being shredded by a black hole. Imagine something tearing our solar system to pieces: sun, planets, and, of course, us.

The pull of gravity decreases with distance. As we get higher and higher above the Earth, the gravitational attraction decreases, but the effect is slow for small masses like the Earth.

Even at the distance of the moon, the force is enough to keep the moon in its orbit and to stop it wandering off into space.

However, the Earth's pull on the moon is a bit stronger on the part of the moon closest to us compared with the other side of the moon. This elongates the moon in our direction by a few metres.

Fortunately, the moon's gravity is strong enough to limit the effect. The moon exerts these tidal forces on the Earth too, pulling the sea up into bumps and giving us the tides.

On the other hand, black holes are a different matter. They are extremely massive and highly compact, as we get closer to them, the difference in gravitational pull between the part of us closer to the black hole and the other side of us becomes enormous, eventually enough to pull us to pieces.

Imagine a star that is on a path taking it close to a black hole. As it approaches, the black hole's pull on the side of the star facing it gets increasingly stronger than its pull on the other side.

This difference leads to the star being pulled out of shape, elongating toward the black hole. As long as the star's own gravity is stronger than this tidal force, the star holds together. However, as it approaches, that tidal force increases and the star starts to come apart.

First to go are its outer layers, where the gravity holding the star together is weakest. This forms a long streamer, flowing inward toward the black hole. As the star gets closer, more and more of its material gets stripped off, until all that is left are shreds of material streaming into the black hole.

This forms a disc around the black hole, referred to, again euphemistically, as an "accretion disc," with the black hole continually nibbling away at the inner edge, and more material spiralling inward to replace it. Some of the material pulled off the star gets flung out into space.

Fortunately, especially for any inhabitants of planets orbiting around stars, like us, these TDE's are rare, maybe one every several thousand years taking place in our galaxy, the Milky Way.

We know of several stars trapped in orbit around the massive black hole in the middle of our galaxy, moving in highly elliptical paths taking them close the black hole.

Their fates are certain, but we cannot predict exactly when. However, when it does happen, observers in other galaxies should be able to see the core of our galaxy brighten significantly. This gives us a powerful tool for looking for TDEs in other galaxies, and greatly increasing the odds of our seeing one.

These days we have telescopes observing thousands of stars looking for exoplanets, and other telescope networks all over the world monitoring large areas of sky. Jan. 29 this year one of a worldwide network of telescopes caught a TDE happening in another galaxy.

The network, known as All-Sky Automated Survey for Supernovae, abbreviated somewhat morbidly as ASASSN, is a set of small (14 cm) telescopes at multiple sites around the globe.

The event was also picked up by orbiting telescopes.

It is easy to conclude the cosmos is full of catastrophic events. However, most of the universe is much more peaceful, like our solar system. It's just that catastrophic things liberate lots of energy, which makes them easy to see.

  • Jupiter and Saturn lie low in the southwest after dark.
  • The moon will be full on the 13th.

Visitors from beyond

Every year, we can count on a comet or two visiting our neighbourhood in the solar system.

For a few weeks these objects can be the most spectacular objects in the sky. These are all orbiting the sun and members of the solar system. However, very rarely a comet visits us from interstellar space; one that originated in another star system.

One of these is moving into the solar system now, and is heading inward towards the sun. It has been named Comet Borisov.

Comets are the result of interactions between lumps of deep-frozen material a few kilometres across, located in Oort Cloud, in the outer reaches of the solar system. They have been orbiting there since the birth of the sun and planets.

On rare occasions two lumps pass very close together. This results in one of the lumps being thrown into a new orbit, taking it inward toward the sun.

The other body in the interaction gets thrown outward, either into a new, more elliptical orbit taking it further out, or maybe out of the solar system altogether. In this case it will wander interstellar space for millions or billions of years until it encounters another star and planetary system.

In space, there is no friction, so the visitor will dive into the system, swing by the star and head off back into interstellar space in some new direction.

Comet Borisov will pass close to the sun on Dec. 7. It is currently moving about 33 kilometres a second and will accelerate to about 44 kilometres a second as it swings past the sun.

Assuming it does not hit anything, it will then vanish back into the space between the stars.

Comets are basically lumps of dirty ice. Anyone who has seen the frozen, dirty slush, loaded with petrochemicals and other organic materials that accumulate on the edges of city streets in Canada in late winter will have seen a good approximation of what comets are made of.

When comets are far from the warmth of a star, they are frozen solid. However, if they are in an orbit taking them close to a star they start to warm up as they approach.

The more volatile materials start to evaporate. Jets of vapour and dust erupt through the comet's surface and because the gravitational attraction of such a small body is very weak, the erupted material escapes into space. 

The sun's radiation and the solar wind push that material away from the comet, forming a tail. Without the tail comets are just small, almost black lumps. With the tail, they become one of the most spectacular objects we see in the sky.

Images taken with one of the Gemini telescopes show Comet Borisov is now developing a tail. The light from the tail bears the signature of organic chemicals.

However, most of us will not be able to observe this comet. The brightest we expect this visitor to be is about magnitude 15, which will place it within the reach of amateur astronomers with fairly large telescopes.

Of course, we could be wrong!.

The presence of these interstellar visitors suggests that, as we would expect, other stars also have Oort Clouds of construction debris surrounding them. 

With all these systems throwing comets inward and outward, there must be a good number of comets moving around in interstellar space, especially since the number is constantly being added to and their chance of destruction tiny.

That raises the surprising issue of why so few of them are visiting us.

One suggestion is that many stars are members of binary or multiple systems, where two or more stars orbit one another, and in the highly disrupted environment caused by the formation of multiple stars, the Oort Clouds would have been disrupted and absorbed by the stars.

This suggests that interstellar visitor comets will be rare, which so far is what we have been seeing. However, since the right search tools have only recently become available, we don't really know.

  • Jupiter and Saturn lie low in the southwest after dark.
  • The moon will reach First Quarter on the 5th.

More Skywatching articles

About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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