Venus and The Moon: Direction from The Heavens

by Paul Kirtley

I was a child in the era of Voyager spacecraft, Carl Sagan's 'Cosmos' TV series and the first Space Shuttle missions. As a result I've been interested in stars, planets and their motion for as long as I can remember.

Later I read of Polynesian navigators using their knowledge of the heavens to guide their boats from one Pacific island to the next. Ever since, I've been fascinated by how this knowledge can be used for wayfinding.

So, whenever something interesting is happening in the sky, I make a conscious effort to relate it to my knowledge of navigation and think about how I might use it to help navigate using only natural cues. I find this is the best way to build up an understanding of natural navigation - over time...

Looking Southwest towards Venus with a crescent Moon alongside, December 26th, 2011

Looking south-southwest towards Venus with a crescent Moon a little further west, December 26th, 2011. Photo: Paul Kirtley.

Venus is the brightest of the planets that can be seen with the naked eye and is the third brightest celestial object after the Sun and the Moon. It alternates between being visible before sunrise and being visible after sunset.

Venus is visible in the evening sky at the moment. As long as your horizon is low enough (it's only reaching a maximum altitude of about 10 degrees above the horizon in the UK currently) and you have a clear sky, Venus can be seen to the southwest from not long after the Sun sets. It will appear as an evening star, before all or most actual stars are visible.

Where I am staying at the moment, there has been a strong westerly wind for the past couple of days and today, the sky has been full of thick, continuous cloud.

Around sunset (on December 26th) the sky cleared somewhat and we were able to see Venus as well as a New Moon a little further west of it.

We know that Venus is, at the moment, visible a little south of southwest not long after sunset but this is by no means always the case. For the time being, however, we can use it to help us find a roughly southwesterly direction.

More generally, it's worth noting that the position of Venus in the sky at a given time of day changes relatively slowly. Tomorrow night it will be not far from where it was visible tonight. And so forth. So, tomorrow night if I see Venus not long after sunset I'll know that I'm looking in an approximately southwesterly direction.

For more information on where Venus will be visible in the sky over the coming weeks and months see here.

Furthermore, observing from where I am tonight, Venus was due to drop below the horizon just before 18:30 hours at 234 degrees relative to true north (data courtesy of the US Naval Observatory). The Sun set at 15:46 hours and, by about 16:30 hours, Venus was clearly visible. At this point, Venus was at about 208 degrees relative to true north. This means that for the whole time Venus was above the horizon and visible, it lay in a direction between 208 degrees and 234 degrees (southwest is 225 degrees).

So, even if I didn't know that it lay in a roughly southwesterly direction, Venus - the third brightest body in the sky after the Sun and the Moon - would have provided a fairly constant point of reference for me.

If I was walking in the dark and kept Venus in a steady position in the sky relative to my direction, I would have walked a relatively straight path and would certainly not have walked in a circle, as we are all prone to do without a frame of reference.

The Moon was also visible this evening. We can also use the Moon to give us an indication of direction. At the time of writing we have a new or waxing Moon, displaying only a thin crescent. You can view the current (time of reading) Moon phase here.

There is no simple way to gain accurate direction from the Moon. We can, however, use one observation of the Moon to help give us a general indication of direction: It's a fairly obvious statement but the reason we can see the moon is because it reflects light.

While the Moon reflects a small amount of light from the Earth back towards us, the brightest part of the moon we can see is reflecting light from the Sun. So it stands to reason that the bright portion of the Moon is facing towards the current position of the Sun.

The photo below was taken about one hour and 20 minutes after sunset. It was taken about 3 minutes before the photo above but on a different exposure setting to let more light into the camera. You can clearly see the light reflecting from the Moon - both from the Sun (the crescent or New Moon) and the Earth (the darker part or Old Moon). You can also see the glow in the region of the sky where the Sun set.

Venus and Moon at 17.09hrs, December 26th, 2011

Venus and Moon at 17.04hrs, December 26th, 2011. Photo: Paul Kirtley.

We all have a good understanding of the motion of the Sun on a daily basis (rises in the easterly portion of the sky and sets in the westerly portion, and completes one revolution in 24 hours.) We can use the Moon to tell us where the Sun is even when we can't see the Sun. And because we know how to use the Sun to give us East, West, North and South, in this way we can also use the Moon to give us an indicator of direction.

You can see in the above photo, taken less than 90 mins after sunset, that the illuminated part of the Moon is facing towards the point below the horizon where the Sun is. Following this line down to the horizon gives a good indication of west.

Alternatively, drawing a line across the "horns" of the crescent moon (which would be at right angles to the line towards the Sun) and extending it to the horizon would give an indication of South. This technique is more accurate the higher the Moon is above the horizon.

Note that the indication of direction we obtained from the Moon ties in with our observations of the position of Venus. This is an important point in all navigation, not just natural navigation; the more directional cues that corroborate, the less likely you are to get lost.

If things contradict each other, then this is the time to be checking everything. If you rely on only one directional cue, then you are more likely to make a mistake.

On this note that we should make the final observation that the clouds were being blown from right to left of the photo. The prevailing wind in this area is westerly and it certainly was a strong westerly today. This also ties in with the indications of direction from our observations of the Moon and of Venus.

If you are reading this on December 27th, 2011, tonight Venus and the Moon will again be in the same part of the sky not long after sunset. This time the crescent Moon will be a little fuller and be sitting above Venus.

If you have a clear sky, this is what you should be able to see: http://www.jodrellbank.manchester.ac.uk/astronomy/nightsky/VenusDec27.jpg

If you have a compass handy, why not check out how the directional cues described above tie in with your compass readings?

Let us know about your observations - of the heavens and/or your compass - in the comments below.

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Paul Kirtley is owner and Chief Instructor of Frontier Bushcraft. He has had a lifelong passion for the great outdoors and gains great satisfaction from helping others enjoy it too. Paul writes the UK's leading bushcraft blog as well as for various publications including Bushcraft and Survival Skills Magazine.

 

{ 4 comments… read them below or add one }

Will

The way I use direction by the moon is if the light on the moon is on the left it is East. On the right, west. Does not work on Full moon though but as it goes east/west in direction, using a fixed point to observe the motion will give you idea of bearing.

Also on Sky And Telescope website it has a Planisphere. Useful for night time navigation.

Reply

Paul Kirtley

Hi Will

Yep that works for the same reasons as the method I was discussing in my blog post. Both the sun and the moon move in an east-to-west direction across the sky. A new crescent (waxing) moon – like the one I photographed the other night – is always close behind the sun, will be seen not long after sunset and will ‘point’ west. An old crescent (waning) moon will be not far in front of the sun and will be seen before sunrise and ‘point’ east.

Because of the east-to-west motion, as you say you can observe the from a fixed point and get an indication of direction. Also, if it is a clear night, just as you can use a shadow stick with the sun during the day you can use the same principle with a full moon at night.

You can also make use of a full moon by remembering it is direction opposite the position of the sun. A rising full moon (not long after sunset) will be in the east and a setting full moon (not long before sunrise) will be in the west. Further, if you know that the local (solar) noon is, for example, 12:10 hours, then the local ‘Lunar noon’ of a full moon will be 10 mins after midnight. At this point the full moon will be directly south…

As I say, I find all of this fascinating :o)

All the best,

Paul

Reply

Christopher Ellis

By looking directly at the Moon, & holding the little finger nail across the horns [ poles] of the moon, then looking across the crook of the elbow, one may have a direct line of sight to the pole beyond the equator, unless one is in the tropics, where either pole can be the result. Get out & try it before commenting.

Reply

Paul Kirtley

Hi Chris,

Thanks for your comment. I think you might have to explain this a bit more before anyone can try it.

Which little finger – left or right? Are you holding your finger straight to your hand? Are you holding your hand straight to your wrist?

Best,

Paul

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