UK ETA NEEDED From January for American Citizens

Thanks for posting. I think your thread is a great reminder that although one may not pass border control on a land crossing into Northern Ireland, there are still requirements.

Thank-you for this. As you mentioned, it’s important to know that even if you are just taking a day trip from Dublin to Belfast, you will need the ETA.

this is the link you need https://www.gov.uk/guidance/apply-for-an-electronic-travel-authorisation-eta

It's possible that the ETA will also apply even if only transiting through Heathrow airport, but details are not clear to me at this moment.
American Airlines uses Heathrow as one of their major stops from the US and connects to locations in Europe .

The ETA does apply to transiting through UK airports. See "What you can do with an ETA" in the up thread government link. Also, Cameron explains it in the second paragraph: https://www.ricksteves.com/travel-tips/trip-planning/europe-visa-waiver

I applied for and received an ETA earlier this week. The process is quick, easy, and the cost with exchange rate was $13.80. This is absolutely nothing compared to the cost of a trip to Europe. My strategy is simply to get it now, it is good for two years, and not think about it again until 2027! To be a bit more specific, this requirement goes into effect January 8, 2025.

An ETA is required for transiting passengers.

Indeed. If you are only transiting, you stillneed an ETA.

I arrive in the UK prior to the 8th of January implementation date, but depart on that day. I haven't been able to find clarification on whether the document is just required for those who enter on or after that day or also applies to trips that extend through or beyond it. Does anyone know?

Only for those who enter on Jan 8 or later.

The ETA gives you authorization to enter the UK. You would have already entered.