Stargazing: Supermoons

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton

On the Nov. 14 we had a Supermoon.

The Moon happened to be Full at the time it was at the point in its orbit where it was closest to Earth. These events are a bit over-hyped because the Moon only looks about 14 per cent larger, which is hard to see without some sort of measuring instrument. However, when we add the Moon looks bigger when near the horizon illusion, the rising of a Supermoon can be spectacular.

The Moon orbits the Earth in about 27 days. Its path is not quite circular. There is a point in its orbit where it is at its closest to Earth (perigee) and on the opposite side of its orbit, a point where it is at its furthest (apogee). Thanks to the Sun and the other planets tugging at the Moon, its perigee varies between 356,400 and 370,000 km, and its apogee between 404,000 and 406,000 kilometres. The most super Supermoon we can get is when the Moon is at its absolute closest, 356,400 km.

The Moon is Full when the Sun, Earth and Moon are roughly in line, so we see the Moon lit full-face with the sunlight coming from behind us. If the three were exactly in line the Moon would pass through the Earthís shadow, and we would be enjoying an ìeclipse of the Moonî.

To get a Supermoon the Moonís orbit has to be oriented so that the Moon happens to be at perigee at the time it is Full. The most super Supermoons happen when the perigee is at the minimum value of its range ñ 356,400km, which was the case on the Nov. 14. Although Supermoons donít look dramatically bigger than other Full Moons there is a more visible consequence: exceptionally high tides.

Our ocean tides are caused mainly by the Moon, with a smaller contribution by the Sun. The Moonís gravity pulls at the Earth and its oceans. Because the ocean on the side of Earth facing the Moon is a little closer to the Moon than the body of the Earth, and is far less rigid, the water is pulled up into a big bump. Similarly, the body of the Earth is closer to the Moon than the water on the far side of the Earth, and is pulled away from it, leaving a second bump of seawater. As the Earth rotates through these bumps of ocean, we experience the tides. The Sun does the same thing, but since it is much further away, the effect is less. When the Sun, Earth and Moon are in line, that is, when the Moon is New or Full, their tidal tugs add together and we get bigger tides, which we call ìspring tidesî. When the Sun and Moon are pulling at right angles, and the Moon is at the First or Last Quarter phases, they partially cancel each other and we get much smaller tides (neap tides). At the time of a Supermoon, the Sun, Earth and Moon are in line, so we get a “spring tide,” and since the Moon is closer to the Earth and its gravitational tug is stronger, we get a bigger one.

The Earth has to move its seabed and continents through these tidal bumps every day, They act as a brake, slowing our worldís rotation, making the day lengthen by about 1.7 milliseconds every century. The Earthís pushing at the bumps drags them forward a little, so they get a bit ahead of the Moon. Just as the Moon pulls at the bumps, the bumps pull at the Moon, so the Earthís dragging them forward is transmitted to the Moon, making it move a tiny bit faster. This acceleration moves the Moonís orbit outwards. As it gets further and further away the Moon will look smaller in the sky. Supermoons will gradually become less super. In 500 million years or so the Moon will have moved far enough away for it to no longer be large enough to cover the solar disc. This will make solar eclipses, one of the most spectacular astronomical events, a thing of the past.

Venus is very low in the southwest after sunset. Look for a very bright, starlike object, shining steadily. Mars is low in the Southwest in the evening. Jupiter rises in the early hours. The Moon will be New on the Nov. 29.

Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory, Penticton.

 

Get local stories you won't find anywhere else right to your inbox.
Sign up here

Just Posted

(Natalia Cuevas-Huaico - Kelowna Capital News)
Titanic was the largest and most luxurious ship in the world. Photo provided and colourized by Jiri Ferdinand.
QUIZ: How much do you know about the world’s most famous shipwreck?

Titanic sank 109 years ago today, after hitting an iceberg

In a feature article published April 10, 2021 in The Times of London, ‘headlined British Columbia has what it takes to rival Napa Valley,’ the valley is praised extensively for its natural beauty and wine. (File photo)
From the U.K. with love: Okanagan wine, scenery receives international praise

The Times of London newspaper recently featured the valley in a wine and travel piece

The currently vacant lot on Main Street where a Dairy Queen, retail and residential spaces have been approved to go into. (Town of Oliver)
Main Street in Oliver is getting a Dairy Queen and residential units

Oliver council gave its approval to fill the empty lot with DQ and residential units

A dumpster was on fire behind a residential complex in downtown Penticton Tuesday afternoon. (Brennan Phillips Western News)
Dumpster fire extinguished in downtown Penticton

There has been a string of dumpster fires lately

A person receives a COVID-19 vaccine at a vaccination clinic run by Vancouver Coastal Health, in Richmond, B.C., Saturday, April 10, 2021. THE CANADIAN PRESS
B.C. sees 873 more COVID-19 cases Tuesday, decline continues

Hospitalizations up to 377, two more deaths for 1,515 total

Restaurant patrons enjoy the weather on a patio in Vancouver, B.C., Monday, April 5, 2021. The province has restricted indoor dining at all restaurants in B.C. due to a spike in COVID-19 numbers. THE CANADIAN PRESS/Jonathan Hayward
B.C.’s COVID-19 indoor dining, drinking ban extending into May

Restaurant association says patio rules to be clarified

Two men walk past a sign on Main Street in downtown Vancouver, B.C., Monday, April 5, 2021. THE CANADIAN PRESS/Jonathan Hayward
Calls for government transparency in COVID data continue as B.C.’s 3rd wave wears on

Social media, where both information and misinformation can spread like wildfire, has not helped

Two men were seen removing red dresses alongside the Island Highway in Oyster Bay. (Submitted photo)
Observers ‘gutted’ as pair filmed removing red dresses hung along B.C. highway

Activists hung the dresses to raise awareness for Indigenous Murdered/Missing Women & Girls

A grey whale off the coast of Vancouver Island is being monitored by Canadian and U.S. researchers, as it has developed lesions after being tagged last year. To try and prevent systemic infection from developing, the team administered antibiotics to the whale on March 31 and April 1. (Photo from the NOAA Fisheries website)
Grey whale off Vancouver Island develops lesions after being tagged, researchers monitor its condition

Canadian and U.S. whale experts administered antibiotics to the animal on March 31, April 1

Sharis Carr, a nurse at the Aaron E. Henry Community Health Service Center in Clarksdale, Miss., holds a box containing doses of the Johnson & Johnson COVID-19 vaccine Wednesday, April 7, 2021. The U.S. is recommending a “pause” in using the single-dose Johnson & Johnson COVID-19 vaccine to investigate reports of potentially dangerous blood clots. (AP Photo/Rogelio V. Solis)
EXPLAINER: What’s known about COVID vaccines and rare clots

These are not typical blood clots – they’re weird in two ways

Marylou Jensen. (Contributed)
Kelowna RCMP searching for missing senior

Marylou Jensen left her Grenfell Road home on foot at 5 p.m.

Skogie’s Express Tunnel Wash on Anderson Way in Vernon. (Submitted photo)
Lawsuit dismissed after vehicle damaged while inside Okanagan car wash

Civil Resolution Tribunal dismisses driver’s claim following a collision inside Skogie’s car wash in Vernon

Most Read