<p>Not within plus/minus a few hundred years at least! The Moon's orbital plane around the Earth (viewed in an inertial frame) rocks back and forth a bit (<a href="https://en.wikipedia.org/wiki/Lunar_precession#Nodal_precession" rel="nofollow noreferrer">Lunar Nodal Precession</a> with a period of about 18.6 years), but that's not going to help here. </p>

<p>You can play with the numbers in this program.</p>

<p><a href="https://i.sstatic.net/DyMxe.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/DyMxe.png" alt="enter image description here"></a></p>

<p><a href="https://i.sstatic.net/pbmLJ.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/pbmLJ.png" alt="enter image description here"></a></p>

<p><a href="https://i.sstatic.net/ARF5P.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/ARF5P.png" alt="enter image description here"></a></p>

<p><a href="https://i.sstatic.net/1Rwsn.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/1Rwsn.png" alt="enter image description here"></a></p>

<pre><code>import numpy as np
import matplotlib.pyplot as plt
from skyfield.api import Loader

degs = 180./np.pi

load = Loader('~/Documents/yournamehere/SkyData')
data = load('de405.bsp')
ts   = load.timescale()

timez = []
dayz  = np.arange(1, 31, 0.1)
dayz  = np.arange(1, 366, 0.1)

#for year in range(1700, 2200):
for year in range(2017, 2028):
#for year in range(2000, 2030):

    #time    = ts.utc(year, 7,  dayz)  # July
    time    = ts.utc(year, 1, dayz)   # January
    timez.append(time)

earth = data['earth']
moon  = data['moon']

RAdegs, Decdegs = [], []

for time in timez:

    RA, Dec, dist = earth.at(time).observe(moon).radec()
    RAdeg, Decdeg = degs*RA.radians, degs*Dec.radians

    # lazy way to block the wrapparound
    dRAdeg = RAdeg[1:] - RAdeg[:-1]
    dropit = np.abs(dRAdeg) &gt; 100
    RAdeg[:-1][dropit]  = np.nan
    Decdeg[:-1][dropit] = np.nan

    RAdegs.append(RAdeg)
    Decdegs.append(Decdeg)

RAdegpts  = [157.5, 157.5+180]  # coordinates from the question
Decdegpts = [-24.0, +24.0] 

if 1 == 1:
    plt.figure()
    for RAdeg, Decdeg in zip(RAdegs, Decdegs): #[::5]:  # plot every 5th year
        plt.plot(RAdeg, Decdeg)
    plt.scatter(RAdegpts, Decdegpts)
    plt.title("2017 throuth 2027 continuously", fontsize=20)
    plt.show()
</code></pre>

CC BY-SA 3.0
2017-06-04T10:27:47.663Z
2017-06-04T10:36:15.257Z
2017-06-04T10:36:15.257Z
uhoh
<p>Title says it, really. As I understand, our total peculiar motion with respect to the CMB is 631 (+/-20) km/h, in the direction of 10.5h RA; -24 deg Dec (or rather away from 22.5h RA; +24 deg Dec, right? That's close to the direction of Lambda Pegasi).</p>

<p>So, if we draw a line from the Moon to Lambda Pegasi, does Earth ever cross it?</p>

<p>I think the answer is no, but don't have enough of a model to prove it to myself.</p>

<p>The inclination of the Moon's orbit is 5.14 degrees, and the Earth subtends about 1.2 degrees from the moon, but the Moon's path extends off at 24 degrees above the ecliptic. So the Earth won't cross it, right?</p>

<p>This is all assuming that the vector info above is the total, and already accounts for the motion of the sun around the milky way. If that's not the case, please let me know.</p>

CC BY-SA 3.0
2017-05-31T18:43:04.313Z
Are you asking whether the Earth ever eclipses Lambda Peg according to an observer on the Moon? The motion of the solar system with respect to the CMB is 370 km *per second* towards Leo, so I'm not sure where you are going with this?
ProfRob
CC BY-SA 3.0
2017-06-02T03:20:50.760Z
Yes, that's an easier way of putting it: does Earth ever eclipse Lambda Peg according to an observer on the Moon?
David Curtis
<the-moon><cosmic-microwave-background><ecliptic>
Does the Earth ever cross the Moon's path with respect to the CMB?
David Curtis
# Does the Earth ever cross the Moon's path with respect to the CMB?

(Asked by: username_0 on 2017-05-31)

Title says it, really. As I understand, our total peculiar motion with respect to the CMB is 631 (+/-20) km/h, in the direction of 10.5h RA; -24 deg Dec (or rather away from 22.5h RA; +24 deg Dec, right? That's close to the direction of Lambda Pegasi).

So, if we draw a line from the Moon to Lambda Pegasi, does Earth ever cross it?

I think the answer is no, but don't have enough of a model to prove it to myself.

The inclination of the Moon's orbit is 5.14 degrees, and the Earth subtends about 1.2 degrees from the moon, but the Moon's path extends off at 24 degrees above the ecliptic. So the Earth won't cross it, right?

This is all assuming that the vector info above is the total, and already accounts for the motion of the sun around the milky way. If that's not the case, please let me know.

**Tags:** the-moon, cosmic-microwave-background, ecliptic

> **Comments:**
> **username_1**: Are you asking whether the Earth ever eclipses Lambda Peg according to an observer on the Moon? The motion of the solar system with respect to the CMB is 370 km *per second* towards Leo, so I'm not sure where you are going with this?  
> **username_0**: Yes, that's an easier way of putting it: does Earth ever eclipse Lambda Peg according to an observer on the Moon?  

---

## 1 answers

### Answer by username_2 on 2017-06-04. Score: 2

Not within plus/minus a few hundred years at least! The Moon's orbital plane around the Earth (viewed in an inertial frame) rocks back and forth a bit ([Lunar Nodal Precession](https://en.wikipedia.org/wiki/Lunar_precession#Nodal_precession) with a period of about 18.6 years), but that's not going to help here.

You can play with the numbers in this program.

[![enter image description here](https://i.sstatic.net/DyMxe.png)](https://i.sstatic.net/DyMxe.png)

[![enter image description here](https://i.sstatic.net/pbmLJ.png)](https://i.sstatic.net/pbmLJ.png)

[![enter image description here](https://i.sstatic.net/ARF5P.png)](https://i.sstatic.net/ARF5P.png)

[![enter image description here](https://i.sstatic.net/1Rwsn.png)](https://i.sstatic.net/1Rwsn.png)

```
import numpy as np
import matplotlib.pyplot as plt
from skyfield.api import Loader

degs = 180./np.pi

load = Loader('~/Documents/yournamehere/SkyData')
data = load('de405.bsp')
ts   = load.timescale()

timez = []
dayz  = np.arange(1, 31, 0.1)
dayz  = np.arange(1, 366, 0.1)

#for year in range(1700, 2200):
for year in range(2017, 2028):
#for year in range(2000, 2030):

    #time    = ts.utc(year, 7,  dayz)  # July
    time    = ts.utc(year, 1, dayz)   # January
    timez.append(time)

earth = data['earth']
moon  = data['moon']

RAdegs, Decdegs = [], []

for time in timez:

    RA, Dec, dist = earth.at(time).observe(moon).radec()
    RAdeg, Decdeg = degs*RA.radians, degs*Dec.radians

    # lazy way to block the wrapparound
    dRAdeg = RAdeg[1:] - RAdeg[:-1]
    dropit = np.abs(dRAdeg) > 100
    RAdeg[:-1][dropit]  = np.nan
    Decdeg[:-1][dropit] = np.nan

    RAdegs.append(RAdeg)
    Decdegs.append(Decdeg)

RAdegpts  = [157.5, 157.5+180]  # coordinates from the question
Decdegpts = [-24.0, +24.0] 

if 1 == 1:
    plt.figure()
    for RAdeg, Decdeg in zip(RAdegs, Decdegs): #[::5]:  # plot every 5th year
        plt.plot(RAdeg, Decdeg)
    plt.scatter(RAdegpts, Decdegpts)
    plt.title("2017 throuth 2027 continuously", fontsize=20)
    plt.show()

```


---

