Caoimhín's Content

Today we are going to take a closer look at a conjunction which is really an occultation, the occultation of Regulus by the Moon. In a piece a little earlier this month I went through several of the close conjunctions, planetary conjunctions and this lunar-stellar conjunction, that are visible during March. There was another piece specifically about the superior conjunction of Saturn with the Sun. Looking at what will be a close conjunction for us, we were able to see that the Moon is occulting Regulus, but Regulus will have already emerged from behind the Moon by the time these stars are actually becoming visible here in Ireland. This is an occultation, but one which from here in Ireland isn’t going to be visible. I want to track down exactly where this occultation will be visible from, and in order to do that I’m going to have to make sure exactly when it starts and finishes from here in Ireland.

Unfortunately will be through to UTC +1 by the 29th of March because it is summer time and we do still use daylight savings time here in Ireland. I’ll only have to adjust by one hour to get the time in the time measurement system, UTC, that is most commonly used internationally for astronomy and many other things. Locally, we’re looking at just 7:04 to see Regulus begin ducking behind the Moon, it’s fully occulted by 7:12. The atmosphere is turned off so we’re not having any interference from the glow of sunset or disappearance of the glow of sunset as it finally goes away, otherwise Regulus would be invisible to us before the occultation. We’ll need to move forward to see Regulus come out the other side. Regulus is fully emerged by 8:15, just starting to emerge at just after 8 o’clock, we’ll say 8:09. That means Regulus is occulted for almost an entire hour, which gives us plenty of time to work with. We want to find a location where the sky has already become completely dark by the time it’s, 6 o’clock UTC, 7 o’clock, using the summer time that we’re using here in Ireland. That means we’re going to have to move in the direction of the dark side of the Earth, past the terminator, outside of the daytime side.

Looking at a Map of the Earth at about 6 o’clock UTC the terminator would be passing through roughly central Europe, with its leading edge just reaching Ireland. Various parts of Europe and Asia would already be in darkness at this time, so that’s the direction that we need to go. We’ll start by hopping a pretty small distance over to the beginning of the Baltic region, around Denmark’s east coast. This should put us in darkness of already, though we may have to jump further to catch the start of the eclipse. Although Regulus is easier to see emerging from behind the Moon, it’s still not visible to the naked eye at the start when it slides behind the Moon. We’re certainly getting closer because Regulus is visible on the western side of the Moon as it emerges. The sky does get dark enough for Regulus to be seen by the time it is emerging out from behind the Moon, but not quite visible as it first gets occulted. Maybe, maybe if it weren’t for the light of the Moon obscuring it. As such, we’re going to have to move a little bit further. We’re going to have to travel further onto the mainland of that Baltic region, towards Estonia, Latvia and Lithuania. I’m trying not to jump in too far simply to make sure that we don’t miss it. From this new location the sky is just barely getting dark enough as the Sun is setting, but even to the naked eye, Regulus is visible next to the Moon, both as separate objects.

We are looking at 55 degrees north, something that I initially assumed wouldn’t matter too much. It shouldn’t matter how far north or south you are, as long as you’re not so far north that the Sun will not have set at this time at all, or so I thought. We have moved to 23 degrees east, so this seems to be a good place to start. From here Regulus is visible getting occulted by the Moon and emerging out the other side. However it looks like Regulus is just cutting over the top of the Moon, it doesn’t seem to be occulted for very long. That difference is possibly because we have come further northward by just a few degrees compared to the location that we were viewing from here in Ireland. This would imply that from further north, Regulus might be visible “above” the Moon. Let’s move further south while trying to remain at the same distance east. Still at 23 degrees east, we’re now only at 44 degrees north, somewhere in Romania. A bit before Regulus’ occultation, the sky is dark, meaning we can move significantly earlier. Indeed, Regulus is visible at just 7:15 for local time, we’re an hour and a half different from UTC. We’re not as far different from the clock time here in Ireland corrected for daylight savings time, but of course, that’s just an extra human complication.

Let’s see how Regulus gets occulted from this location. It’s starting to get occulted at, we’ll say 8:15 local time for when the occultation begins. Regulus does seem to pass almost through what you’d call the Moon’s center before coming out the other side at about 9:30. That is a longer occultation, it’s closer to an hour and a quarter of an occultation, rather than the just about an hour’s worth of an occultation that is invisibly occurring here in Ireland. That does seem to be due to how much lower on the planet we are. We are a little bit closer to the equator, so Regulus is more directly behind the Moon. Parallax does make a difference, even if you’re only moving across the Earth’s surface. This looks to be just about the “first” place on Earth where it will be possible to see the occultation from start to finish, first as in earliest local time. This is visible just as the Sun is setting. That means that a little bit before the occultation, Regulus wouldn’t be visible in the sky due to the sunlight. If we move to the other side of night time, to the opposite extreme, then we should be able to see where this occultation is visible to be ending just as the Sun rises, rather than starting as the Sun sets.

It looks like it is not visible in Japan. It looks like the Moon has already set by the time the occultation begins, so we have moved too far. We’ll come back onto the mainland, pretty close to the Korean peninsula. That certainly makes things a little bit better. The occultation still has not begun while the Moon is above the horizon, but the Moon still drops under the horizon before the occultation begins. We have to go a little bit further back into the mainland of this region, roughly Northern China/ Southern Russia, around the border of Mongolia. In Stellarium, none of the borders are written displayed on the world map, so it can be a little bit tough to tell exactly what country you’re in, especially if you don’t recognize many of the names of local cities displayed. Regardless, from roughly this area, the Moon and Regulus are up for the whole duration of the occultation, but the Moon misses Regulus completely. It looks like we have perhaps moved too far north, showing again the importance of parallax just. This occultation might only be visible from a more narrow region of the Earth, and on a more slanted path. Thanks to the Earth’s tilt, the ecliptic doesn’t line up perfectly with the Earth’s equator. The motion of the Moon through the sky follows the Earth’s equator, so it doesn’t line up perfectly with the ecliptic. It looks like further north it also isn’t visible, so we need to go further south. Tracking this occultation was a little bit tougher than I thought it would be.

Moving a little further south and taking a closer look, Regulus still misses the Moon. It’s still passing what appears to be over the Moon, even from this much more southerly latitude. We’ll try going further south just in case this occultation is visible on an angled line going across the equator, which seems to be the case. From a bit further south, we can only see the occultation with the ground removed, so we’re going to have to move back towards the west as well. We’re going to move to off the southern coast of India, a little east and south of where the Maldives would be. That lets us see the Moon and Regulus being occulted by it. We’re only seeing the beginning of the occultation though, not the end. It’s still too close to the horizon. We’re narrowing it down, we’re definitely getting close to the location where it would be visible beginning and ending. Moving a little bit further to the north, we can still catching the occultation now that we’ve moved a little bit further to the west. From roughly northern India, possibly a little north of India, it is visible. The Moon does pass close to the top of the Moon, making the occultation shorter, but visible from start to finish.

It looks like from around the Indian subcontinent, down towards, though not all the way to, but towards Indonesia and Maritime Southeast Asia, up towards northern Europe, but not as far as Ireland, that’s where this occultation is going to be visible. We’ll split the difference just to double check, we’ll come up to around the Caspian sea and see what we can see. We can see the occultation, from start to finish. Not perfect, Regulus is still definitely going across what seems to be the top of the Moon, but seeing it from start to finish reasonably close to midnight.

That is the occultation of Regulus by the Moon. Barely visible ending from here in Ireland, just about visible beginning from the far east, you can see it about to be beginning at least, but really visible from what seems to be quite a bounded area, quite a small area from sort of Eastern Europe, northeastern Europe down towards, down towards the Indian subcontinent. This likely includes North Africa and the Middle East. I hope that you enjoyed this piece, if you did then please do like it.If you like this kind of content, then please subscribe to this website and my YouTube channel. Thank you very much for reading and hopefully I’ll see you back here next time.