Chances are you may not have heard of the wonderfully named Sinus Viscositatis, the Bay of Stickiness, its name only being officially sanctioned by the International Astronomical Union on 30 December 2022.
Sinus (from the Latin for bay) is not a strictly geological term and in the context of the Moon it refers to features that are basically small plains.
Possibly the bay that we’re most familiar with is the semicircular feature on the northwestern shore of 1,250km Mare Imbrium, namely 400km x 260km Sinus Iridum, the Bay of Rainbows.
For more lunar observing, read our pick of the best features to observe on the Moon and underrated features to observe on the Moon
Quick facts
- Size: 68km x 50km
- Longitude/latitude: 41° W, 35.2° N
- Age: Around 3.9 billion years
- Best time to see: Three days after first quarter or two days after last quarter
- Minimum equipment: 100mm telescope
How to find Sinus Viscositatis
Being a large feature, Sinus Iridum is easy to see using binoculars and it’s a distinctive part of the Earth-facing side of the Moon.
Sinus Viscositatis is significantly smaller and easily overlooked, but Sinus Iridum is a good guide to finding it.
The best time to see Sinus Viscositatis is when the terminator is also optimal for Sinus Iridum.
The southern end of Sinus Iridum is marked by a rugged outcrop known as Promontorium Heraclides.
This also doubles as the clair-obscur effect known as Cassini’s Moon Maiden, which appears four days after first quarter and is supposed to resemble a girl sitting on a rock looking out across the bay.
Keep heading south from Promontorium Heraclides, along the elevated highland material that borders Mare Imbrium, and it’s at the southern end of this region that you’ll find Sinus Viscositatis.
Observing Sinus Viscositatis
It’s a curious feature that looks vaguely rectangular in shape, the long axis aligned south-southwest to north-northeast.
The south-southwest edge is open and level with the lava boundary floor of 700km x 500km Oceanus Procellarum.
The north-northeast edge is open too, a narrow channel of lava heading north-northeast to connect to other unnamed regions of similar flat lava.
The channel tracks between two very distinctive lunar domes: Mons Gruithuisen Delta to the east and Mons Gruithuisen Gamma to the west, believed to have formed from thick, viscous silicic lava.
As this material flowed, its thickness prevented too much sideways movement and the domes were formed.
In contrast, the surface of Sinus Viscositatis is made of basaltic lava which, being thin and runny, created the large expanse of flatness.
Mons Gruithuisen Delta rises to a height of around 1.8km and has a diameter of approximately 27km.
Mons Gruithusien Gamma is smaller at 20km wide and 1.5km high.
Its shape has earned it the less-than-flattering description of looking like an upturned bath tub!
If you fancy a challenge, there’s a tiny 2.9km craterlet in the central plateau peak of Mons Gruithuisen Gamma.
The crater off to the southeast of Mons Gruithuisen Delta is 9.4km Gruithuisen B.
The southern portion of Sinus Viscostatis is marked by another dome: 26km x22km Mons Gruithuisen Zeta which appears very pockmarked.
The western corner of Sinus Viscostatis is marked by the concentric 6km crater Gruithuisen K.
This is a tough feature to see and an interesting target for high-resolution imaging setups, the inner ring of Gruithuisen K being just 1.7km in diameter.
Have you photographed or observed Sinus Viscositatis? Let us know by emailing contactus@skyatnightmagazine.com