BBC1 programme "Wonders of the Moon"

[Klitos]

Has anyone seen Wonders of the Moon on BBC1 on Tuesday 30 Jan 2018? It was timely, as people were getting excited about Wednesday's "supermoon", and was aimed at a lay audience, so was good in that it raised the public's enthusiasm towards astronomy. Unfortunately, I noticed the following howlers, and I'm sure there were many I've missed:

1. The orbit of the moon is not circular; it's oval.

I would have excused that inaccuracy as not everybody would know what "elliptical" means. However, they then illustrated the orbit with a ellipse (well done!) but unfortunately place the earth at the centre of the ellipse!

2. The moon is nearly one third brighter when it's a supermoon.

Let's do the maths:
Distance at perigee: 362,600 km
Distance at apogee: 405,400 km
Distance ratio: 405400/362600 = 1.118
Brightness ratio: 1.118² = 1.25

So the moon is only 25% or one quarter brighter when closest to us than when at its furthest.

So the programme was good overall for raising public interest, but why the exaggerations? Surely they could still make it interesting while maintaining accuracy.

[Carole]

I didn't see it, but you would think they could get their facts correct.  We have come to expect the news presenters to get things a bit exaggerated but a programme dedicated to the Moon should have correct information.

Carole

[Mac]

Wednesdays moon was NOT a super moon, that was on the 1st of Jan,
Plus the moon in August is closer then Wednesdays one. 

Code Select Expand

2018         Jan 01  21:54   356566 m      Jan 15  02:10   406461 M     
             Jan 30  09:54   358995        Feb 11  14:16   405701       
             Feb 27  14:48   363938        Mar 11  09:13   404682       
             Mar 26  17:17   369104        Apr 08  05:32   404145 m     
             Apr 20  14:44   368713        May 06  00:35   404458       
             May 17  21:06   363777        Jun 02  16:34   405316       
             Jun 14  23:55   359507        Jun 30  02:43   406061       
             Jul 13  08:28   357432        Jul 27  05:44   406223       
             Aug 10  18:05   358083        Aug 23  11:23   405744       
             Sep 08  01:21   361355        Sep 20  00:54   404875       
             Oct 05  22:29   366396        Oct 17  19:16   404227       
             Oct 31  20:05   370201 M      Nov 14  15:57   404341       
             Nov 26  12:10   366623        Dec 12  12:25   405177       
             Dec 24  09:52   361060                                    

Mac.

[Mac]

I think they were using the maximum and minimums that are theoretically possible.

Perigree 2052 Dec 6 356425km
Apogees 2016 Dec 12 406709

Difference 406709/356425 = 1.14107
Squared = 1.302

30% near enough to 33%

Mac.

[Klitos]

Mac,

How does that work? I've never known the size or eccentricity of an orbit to change over time (unless a force is applied, as it is with the Space Station).

Is it because sometimes perigee is over Mount Everest while at other times it's over the deepest ocean, so the distance is measured from different points?

[Carole]

Does it depend on the position of the Sun at Moon perigee and maybe Earth's own aphelion or perihelion having an effect on the Moon's position?

Carole

[Rick]

So the programme was good overall for raising public interest, but why the exaggerations? Surely they could still make it interesting while maintaining accuracy.

Too few scientists involved in production, I'd guess, quite possibly made worse by some final decisions been taken by someone who knows plenty about making TV programs but diddly-squat about the science...

[Klitos]

Does it depend on the position of the Sun at Moon perigee and maybe Earth's own aphelion or perihelion having an effect on the Moon's position?

Carole

I thought you'd hit the nail on the head there, Carole. However, it doesn't seem to be the case.

Mac quoted the most extreme apogee/perigee dates as Dec 6 and 12 (in different years). Both of those dates correspond to full or nearly full moon. Therefore:

1. The sun's position relative to the moon is about the same on both dates.

2. The earth is at approximately the same point in its orbit on both dates (both in December). Perihelion is on January 3, year after year.

So that theory doesn't hold, though it was the most plausible explanation so far.

[Carole]

Thanks for doing the research Klitos, it was just a guess on my part, but I couldn't be bothered to do the research lol.

Carole

[Rick]

I don't remember hearing the term "supermoon" used until relatively recently. It's a term used mostly in media hype. A little googling turns up a couple of pages that explain it:

From https://www.timeanddate.com/astronomy/moon/super-full-moon.html

Supermoon is not an official astronomical term. It was first coined by an astrologer, Richard Nolle, in 1979. He defined it as 'a New or a Full Moon that occurs when the Moon is at or near (within 90% of) its closest approach to Earth in its orbit'. It is not clear why he chose the 90% cut off in his definition.

From http://earthsky.org/space/what-is-a-supermoon

A supermoon is a new or full moon closely coinciding with perigee, the moon's closest point to Earth in its monthly orbit.

From https://en.wikipedia.org/wiki/Supermoon

A supermoon is a full moon or a new moon that approximately coincides with the closest distance that the Moon reaches to Earth in its elliptic orbit, resulting in a slightly larger-than-usual apparent size of the lunar disk as seen from Earth.

[Mac]

The moon follows an elliptical orbit around the earth,
this means that the earth is at one of the two focus points.
If it was circular then there would only be one focus point and the distance would not change.

so the distance between the earth and moon varies over time,
depending on where it is in its orbit.

However because we are at a focus point, the distance from the earth to the moon on either side of this elipse is different.

so when its at its furthest points on both sides the distances are different.

The moons siderial month is 27.3 days which is the time taken for the moon to return to the same position in the sky relative to the stars.
Its synodic time is 29.5 days, which is the time between the full moons,

So as the moon goes around the earth the distance between the full moons will change on each full moon.
So when the moon is at the closed point it will be the supermoon

Now why does this change and give us different values every year for the closest and furthest values.

This is because the earth is also in orbit around the sun, again in an ellipse, the force of gravity from the sun is also acting on the moon as well as the force from earth,
so when we are at our closet approach to the sun, the sun will impart more force on the moon, which will in turn slightly alter the orbit of the moon,
and when we are at the furthest point from the sun, the force is less on the moon.

So as we orbit the sun and the moon orbits us, the gravitational forces on the moon from both us and the sun cause it to alter its orbit slightly over time.
and vice versa.

Its called gravitational wobble, its also what was used to detect the presents of Neptune.

Mac.

[Carole]

That's a more detailed explanation for what I was saying. 

I don't remember hearing the term "supermoon" used until relatively recently. It's a term used mostly in media hype.

That's what I thought too.

Carole