LEGO Technic 42179 Planet Earth and Moon in Orbit – Finally an orrery! [Review]

As time goes by and LEGO Technic adds more and more elements to its repertoire, it becomes more and more of a force to be reckoned with in the world of STEM toys. Nowhere is this more evident than in the recent addition to their lineup: the LEGO Technic 42179 Planet Earth and Moon in Orbit. Some form of orrery (a model of the orbit of planets and moons around the sun) has been on many wish lists for a long time, and finally the wait is over. The kit will be available March 1st and retail for US $74.99 | CAN $99.99 UK £69.99. Join us as we build the 526-piece model and see if it’s everything we’ve hoped for. 

The LEGO Group sent The Brothers Brick an early copy of this set for review. Providing TBB with products for review guarantees neither coverage nor positive reviews.

Unboxing the parts, instructions, and sticker sheet

The box is on the smaller side. The front features the new “SPACE” banner with the classic logo, while the back shows how the model works.

Inside are 4 numbered paper bags and an unnumbered plastic bag. Loose are the new sphere halves for the sun and a couple of large Technic rings. The numbered bags have smaller numbered bags within them, all with their own QR codes and part numbers printed on them. It’s cool to see the switch to paper finally happening.

Because this is a basic Technic set and not an exclusive 18+ set, the instructions don’t have the details on the first several pages that we’ve become accustomed to seeing in those. It’s unfortunate, because this seems like a situation that would really benefit from some interesting information about the subject matter.

As for the stickers, we get a small sheet that includes the months and moon phases.

The build

First order of business is building the base for the orrery. It includes large black banana gears and heavy molded wheels that we haven’t seen for over a year. The center ring is in that lovely new red-orange color.

From there, we quickly start building a mechanism that includes a series of gears. The large banana gears that compose the outer ring are simply to provide a stable base, but are not used at all in the mechanisms.

Throughout the model gears are strategically placed in order to create a system that is being geared down (slowed from input to output). In the GIF below, you can see the beginnings of this system. A 36-tooth gear at the bottom will serve more than one purpose. Here, we have roughly a 5:1 gear ratio.

The sun will sit upon a framework that includes a large turntable.

From the previous group of gears, the top drives the turntable (by a further reduction of over 1.6:1), while the bottom is connected to an axle and gears that come up through the center to rotate the sun. In essence, the sun is being spun while also rotating, but the turntable (rotating) portion’s greater purpose is actually to rotate the large arm that the Earth is attached to. At this point a bevel gear also transfers the the motion perpendicular to the crank, which, of course, is actually the driver of the entire string of mechanisms. The GIF below is as fast as I could spin the crank, multiplied by 2.5.

Next, the upper ring is added and slopes are used to adorn the sides while also providing support. Beauty is sort of in the eye of the beholder here. Some might like that the core gets a bit covered, while others might wish all the gears were exposed for viewing.

After that, we turn our attention to the large arm. A pearl gold connector is used as a guide to provide stability, and we’ll come back to that in a moment.

The mechanism that sits just below the sun needs to transfer rotation on a perpendicular axis, while also going around that axle to feed motion to the sun. Bevel gears are used to accomplish this and a couple others are used to increase the input speed by a ration of 1:3. A small turntable at the top does not move but simply allows for the axle to pass through the center while still providing support underneath.

With that, we attach this chunk to the base and apply the month stickers to the upper ring.

The sun is added next. It’s composed of two new bright light yellow 11×11 sphere halves.

Once again, the GIF below shows an up-close look at the mechanism. Like the above, this is as fast as I could turn the crank times 2.5. December to February actually takes at least 12 seconds.

The Earth and moon part of the mechanism looks like it belongs inside a clock.

This is where it becomes even more complex and intriguing, with more than one things happening at the same time. Like the sun, we have both spin and separate rotation at play.  There is a gear reduction rotating the moon, and a gear overdrive (increase in speed) rotating the Earth.

The moon is represented by a little light bluish grey ball joint, while the Earth is made by a smaller version of the sun sphere (5×5), printed with the continents. A universal joint allows it to be tilted on an angle, just like the real thing. Additionally, a pearl gold tooth marks the moon phases.

While building this set, you do realize the importance of closely following the directions to make sure that everything lines up correctly. Fortunately there is a helpful diagram in the instruction manual for the final connections.

The completed model

With that, the orrery is finished! It truly is a marvel of LEGO construction. You can see from the photo and GIF below that there is the slightest bow to the arm. It’s not bowing the pieces, just using every bit of leeway there is in the connection points. The arm is relatively heavy, and the guide support that was mentioned early on is certainly needed. Regardless, it looks pretty nice and is compact enough to fit on a shelf for display.

Like the other GIFs, the one below is sped up significantly. Even with the use of a motor it takes several seconds to complete a revolution. Just like it should!

Conclusions and recommendations

You might be wondering, is it accurate though? And I’m glad you asked, but I cannot provide a 100% straightforward answer. If you make extra sure to line everything up perfectly and turn the crank at a reasonable speed without getting wild, yes it is surprisingly accurate. Considering it’s made within the constraints of LEGO, this is a true feat that is commendable. But there is give within the system. If you’re a couple gear teeth off in your final placement you could be the tiniest bit off. If you spin the crank as fast as possible, there might be a bit of slippage. The largest margin for error is if you mess with the Earth directly, trying to spin or rotate it by hand. This can create variability. Over time even if you’re kind to it, it might fade out of accuracy. However, generally speaking, I would give it good marks.

Some of us have asked for an orrery for a long time. A small few have even made their own. It was high time that LEGO designers gave it a whirl (no pun intended), and I would say they didn’t disappoint. Overall it’s a fun model to both build and admire. While an 18+ model would’ve been really cool to see, I’m not disappointed at the younger age suggestion. It feels like a fun kit for any experienced builder to tackle, and I would recommend it highly to anyone with interest.

If you like Technic and/or Classic Space, you’re in for a treat! We have loads of reviews coming down the pipeline within the next week. Stay tuned and follow along in our reviews archive!

LEGO 42179 Planet Earth and Moon in Orbit will be available starting March 1st and retail for US $74.99 | CAN $99.99 | UK £69.99.

The LEGO Group sent The Brothers Brick an early copy of this set for review. Providing TBB with products for review guarantees neither coverage nor positive reviews.

16 comments on “LEGO Technic 42179 Planet Earth and Moon in Orbit – Finally an orrery! [Review]

  1. R0Sch

    I think you made a mistake with a gear pair, because the sun should rotate counterclockwise, same way as the earth and moon orbit around it in counterclockwise direction. All the other videos and official LEGO animation show this as well.
    P.S. The movement and orbital periods are extremely accurate for a LEGO model. The Earth completes 365.5 days in one complete orbit and the moon has a 27 day orbit on this model.

  2. Bre Burns Post author

    @R0Sch, The rotation is a matter of what way you turn the crank. So if it was going the wrong way in my GIFs, it’s because I was so focused on the mechanism that I didn’t realize I was turning it the wrong way. Thanks for pointing that out though for anyone else who might have picked up on that and was confused.

    Edit: I realize what you’re referring to in the last GIF. It actually looks like it’s going the opposite direction because of a combination between how fast I have it turning and the framerate of the GIF. Like a car wheel, when some go fast, it looks like they’re spinning in reverse. Hope that makes sense.

  3. Colin Musgrove

    Will Lego be making add ons so you can build up the whole solar system if not could they im loving the look of this set but i feel Lego are missing out on such an opportunity.

  4. Filip Kljaić

    My I ask something unrelated?! Why never prices in €? You put prices for the UK but not for 20+ countries in Europe?

  5. Bre Burns Post author

    Hi Filip! That’s a good question. The reason is two-fold. Firstly, we provide links to the three places with the highest concentration of our readers. Secondly, the way the links are generated would mean that we’d need to provide 20+ links to go to all those countries individually. Apologies if this is an inconvenience.

  6. Jimmy

    It’s not an Orrery – it’s a tellurion or tellurium as it’s only the Earth, Sun and Moon… It has to have all the planets of the Solar System to be an Orrery.

  7. Bre Burns Post author

    @Jimmy, you’re half right. A tellurium is a type of Orrery. A tellurium includes only the Sun, Moon, and Earth, while an Orrery includes AT LEAST the Sun, Moon, and Earth (but may or may not include more planets). Many of the early Orrery models only included the three. I chose to use the word that is most broadly known. Source: an Astronomer.

  8. Haoyang Wang

    It takes 121.5 cranks for the earth to make one circle around the sun.
    The earth self-rotate 3 times per crank.
    1 year = 121.5 x 3 + 1 = 365.5 days

    It takes 9 cranks for the moon to make one circle around the earth, but this does not mean a lunar month = 3 x 9 = 27 days, because a month is defined as the duration between 2 new moons, and as the earth moves along the orbit, it takes slightly more than one circle for the moon to reach her next new-moon position.

    1 year = 121.5/9 – 1 = 12.5 lunar months
    1 lunar month = 365.5 / 12.5 = 29.24 days
    According to it’s 29.53059 days. Pretty close.

  9. Haoyang Wang

    That is, if it works as intended.
    Each year (i.e., when the earth makes one circle around the sun) the 20-tooth black gear at the center axle (i.e., the gear installed at Step 91) “loses” one turn due to the relative motion. As the result, the moon loses 1/3 circle and the earth loses 9 days per year.
    (I verified the moon. Haven’t count the earth days.)

  10. Richard Brown

    @Haoyang, the moon and the earth rotation are geared off the same driveshaft, and so new moon to new moon (e.g. sticker to sticker) is exactly 27 rotations of the earth if you set the frame of reference to be the orbiting module

    But we know each rotation of the earth in that frame is 365.5/364.5 days, so the new moon to new moon (synodal) month is 27*365.5/364.5 = 27.07 days

  11. Haoyang Wang

    New moon happens when the moon is in line with the sun and the earth. New moon to new moon is more than 27 earth-rotations.
    Your equation should be 27*365.5/(364.5-27) = 29.24 days

    These day and month ratios are impressively accurate, only to be marred by the off-by-one error in the central gear. Sigh.

  12. Haoyang Wang

    You can roughly check the month length by counting the number of cranks between one new moon to the next.

  13. John Armstrong

    Wow – as I read the comments – wow. How do you get used the attempts at corrections on words and timings and gear ratios – blah blah blah. Please, what an awesome, cheap for what it is, depiction of the sun, earth and moon interrelationship – will keep kids entertained and give them some concrete picture of what’s happening. As a teacher, I’m thinking how do I use the wow factor for kids to help them conceptualize phases of the moon, eclipses, etc – Great Job – on my way to get one and I’ve already built JK Brickworks’ model.

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