Here’s why the Jaffa cake eclipse advert is so very wrong

Full moon. Half moon. Total nonsense.

It was 1999 when McVitie’s famous eclipse Jaffa Cake advert came out.

If you’re over 30, you know the one.

‘Full moon. Half moon. Total eclipse.’

Oh how we laughed as we did the same, destroying a pack of Jaffa Cakes with astonishing ease while copying the teacher as she taunted her young students.

But it was wrong. So wrong.

Not because of the calories – although the doctor who was slammed this week for telling us not to eat our Easter eggs in one go would not approve.

No, scientifically, it’s all wrong – academic malpractice of the highest order. And it doesn’t seem anyone has ever really noticed.

But now, sandwiched between a lunar eclipse and a solar eclipse, seems as good a time as any to blow this two-decade wrong wide open.

When the Moon is eclipsed, it doesn’t disappear.

It turns blood red.

So more of a raspberry Jaffa Cake really, but nobody likes those.

You see, while it’s true that during a lunar eclipse the Earth completely blocks the Sun from the Moon, that doesn’t mean no light is reaching it. Thanks to the Earth’s thick, life-supporting atmosphere, light from the Sun that passes through it is bent towards the Moon, a phenomenon known as refraction. 

And if you had a better teacher than Ms Jaffa Cake at school, you may have learnt that when light is refracted, it creates a rainbow, splitting visible light into all its component colours.

This happens during the lunar eclipse too, and only red light is focused on the Moon.

When we really can’t see the Moon, that’s a new moon. Here, the Moon is between the Sun and the Earth (although not directly, that’s a solar eclipse), so the side facing us is thrown into darkness.

And while we’re on the pedant express, a half moon doesn’t come before a total eclipse.

In fact, there’s technically no such thing as a half moon. What the Jaffa cake shows is a crescent moon, which we see twice a month as it travels around Earth. 

Imagine the Sun is at 12 o’clock, then the crescent moon is visible either side of noon, from after 9 o’clock until just before 3 o’clock.

At 9 and 3 it becomes a quarter moon, despite looking like a perfect half. The full moon is at 6 o’clock, when directly opposite the Sun – unless the Earth is directly in between, in which case there’s a lunar eclipse.

To be honest, the whole Jaffa cake thing is a mess and a shockingly bad science lesson. Not only that, but last year McVitie’s doubled down on the whole thing, revisiting the advert and dragging national treasure Bonnie Tyler into the whole debacle. 

But we get that full moon, crescent moon, new moon is neither catchy nor particularly funny. 

And, of course, we’re joking.

Obviously the real Jaffa cake scandal is the rampant shrinkflation the cakes (not biscuits) have undergone since those heady days of the eclipse advert.