Does the moss in Westeros tend to grow on the south side of the trees?

Question

In A Storm of Swords Arya is running with Gendry through a forest and is telling the direction via the moss

"But there's no sun. How do you know?"

"From the moss. See how it grows mostly on one side of the trees? That's south."

- A Storm of Swords, Arya I

And later

On the morning of the third day, Arya notice that the moss was growing mostly on the wrong side of the trees. "We're going the wrong way," she said to Gendry, as they rode past an especially mossy elm. "We're going south. See how the moss is growing on the trunk?"

- A Storm of Swords, Arya III

I know that in reality moss doesn't only grow on one side of the trees, but in the northern hemisphere it tends to grow more heavily on the north side of the trees, as the sun is angled slightly southward due to the axial tilt of the Earth causing more shade to be on the north side of the tree. I am unfamiliar with how it grows in the southern hemisphere as I have never visited.

As posted in the comments here is a link to a question about moss and which side of the tree it grows on where.

Since the further north you go in Westeros the colder it gets, and the south is a desert type region that is also warm, I would postulate that they are also in the northern hemisphere of their planet. As such, would the moss not then grow on the north side of their trees as well, not towards the south as Arya is suggesting?

Answer 1

We Do Not Know

The quotes are left vague on purpose as to which side of the tree the moss is actually growing on.

"From the moss. See how it grows mostly on one side of the trees? That's south."
_{A Storm of Swords, Arya I}

Could indicate that Arya is pointing, that she means the uncovered side is south or it's north we don't know.

The same can be said for your second quote. There are a couple of other quotes that are all intentionally vague as well. This one seems to indicate again that it is only mostly one side and not always just the one side:

"Are you sure we're going north?" asked Gendry. "All these hills . . . if we got turned around . . ."
"The moss on the trees—"
He pointed to a nearby tree. "That tree's got moss on three sides, and that next one has no moss at all. We could be lost, just riding around in a circle."
_{A Storm of Swords, Arya I}

The only quote that tells us for definite which side the moss is growing on is the following:

Deepwood's mossy walls enclosed a wide, rounded hill with a flattened top, crowned by a cavernous longhall with a watchtower at one end, rising fifty feet above the hill. Beneath the hill was the bailey, with its stables, paddock, smithy, well, and sheepfold, defended by a deep ditch, a sloping earthen dike, and a palisade of logs. The outer defenses made an oval, following the contours of the land. There were two gates, each protected by a pair of square wooden towers, and wallwalks around the perimeter. On the south side of the castle, moss grew thick upon the palisade and crept halfway up the towers. To east and west were empty fields. Oats and barley had been growing there when Asha took the castle, only to be crushed underfoot during her attack. A series of hard frosts had killed the crops they'd planted afterward, leaving only mud and ash and wilted, rotting stalks.
_{A Dance with Dragons, The Wayward Bride}

However, this is on a castle and not a tree and so the south side could give more shade than the other sides or be damper etc. We don't seem to have enough information to go on.

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Since the further north you go in Westeros the colder it gets, and the south is a desert type region that is also warm, I would postulate that they are also in the northern hemisphere of their planet.

We simply don't know where Westeros is on the planet so we can't just guess at that. The seasons are also different and north of the Wall the lands are always winter. By going off temperatures that change so rapidly over such a small island like Westeros it is impossible to tell where on the planet it is.

Answer 2

There seems to be no logical explanation for this. If moss works remotely the same on Planetos as it would on Earth, it should grow on the north side.

Moss on which side?

On Earth, moss tends to grow on the north side of trees, because this is in the shadow. This prevents water from evaporating, generating a nice moist place for the moss to grow. The reason moss needs this is that it doesn't have roots like most other plants, so it can't suck up water from the ground.

Moss on south side implies the sun arches through the north, instead of through the south

In the Northern Hemisphere here on Earth, the sun passes from East to West, through the South. This causes the north side of trees to be in the shadow more than the southern side.

If moss grew the opposite direction, logically the sun should reflect that difference. So does it?

It does not

There are a multitude of quotes that the sun does indeed rise in the East and set in the West, but there is no direct information on whether it stands in the south at midday. However, we can judge the direction by the shadows cast by the sun:

The Night's Watch permitted the forest to come no closer than half a mile of the north face of the Wall. The thickets of ironwood and sentinel and oak that had once grown there had been harvested centuries ago, to create a broad swath of open ground through which no enemy could hope to pass unseen. Tyrion had heard that elsewhere along the Wall, between the three fortresses, the wildwood had come creeping back over the decades, that there were places where grey-green sentinels and pale white weirwoods had taken root in the shadow of the Wall itself, but Castle Black had a prodigious appetite for firewood, and here the forest was still kept at bay by the axes of the black brothers.
_{A Game of Thrones - Tyrion III}

We have the Wall, which runs in a straight line from West to East (or East to West, depending how you look at things). If the sun stands in the south, the shadow is north of the wall, if the sun stands in the north, the shadow is south of the wall.

Given that there are trees growing in the shadow of the wall on the northern side, this shows that the sun stands in the south, just like ours.

Additionally, the opposite would imply that Castle Black would be almost permamently in the shadow of the wall. That sounds cold.

Conclusion

The sun on Planetos (is it really on Planetos?) follows a similar path as ours, so logically the moss should grow on the same side as on Earth.

Caveats:

• It could be that somewhere in the middle of Westeros is the equator, meaning that south of it the moss would be the other way around. The quote from TheLethalCarrot would imply that for that to hold the equator should be north of Deepwood Motte, which is pretty damn far north.

• This does assume nature follows similar logic as on Earth. This is an assumption. _{Hi Edlothiad}